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Photomultiplier 
Manual 




Theory Design Application 



Technical Series PT-61 



Q Photomultiplier 
U Manual 



The purpose of this Manual is to help designers and users of electro- 
optical equipment using modern photomultiplier tubes achieve a better 
understanding of these devices. The Manual includes information on 
the construction and theory of operation of photomultipliers together 
with discussions of photomultiplier characteristics and the methods used 
to measure these characteristics. Information on typical applications 
is given along with basic data dealing with the measurement of light and 
radiant energy, noise statistics, spectral matching factors, and other 
topics as shown in the Table of Contents. Finally, a data section is 
included that gives typical characteristics of RCA photomultipliers. 



RCA I Electronic Components I Harrison, N.J. 07029 



Copyright 1970 by RCA Corporation 
(All rights reserved under Pan-American Copyright Convention) 

Printed in U.S.A. 9/70 



Contents 



Introduction 3 

Photoemission and Secondary Emission 5 

Principles of Photomultiplier Design 19 

Basic Performance Characteristics 30 

Statistical Fluctuation and Noise 56 

Application of Photomultipliers 77 

Voltage-Divider Considerations 103 

Photometric Units and Photometric-to-Radiant Conversion . . 112 

Radiant Energy and Sources 119 

Spectral Response and Source-Detector Matching 127 

Technical Data 134 

Outlines 161 

Basing Diagrams 174 

Index 183 



Information furnished by RCA is believed to be accurate and reliable. 
However, no responsibility is assumed by RCA for its use; nor for 
any infringements of patents or other rights of third parties which 
may result from its use. No license is granted by implication or 
otherwise under any patent or patent rights of RCA. 



Introduction 



PHOTOMULTIPLIERS are ex- 
tremely versatile photosensitive 
devices that are widely used to de- 
tect and measure radiant energy in 
the ultraviolet, visible, and near- 
infrared regions of the electromag- 
netic spectrum. In most of this range, 
photomultipliers are the most "sensi- 
tive" detectors of radiant energy 
available. The basic reason for this 
superior sensitivity is the use of sec- 
ondary-emission amplification which 
makes it possible for photomultipliers 
to approach "ideal" device perform- 
ance limited only by photoemission 
statistics. Other outstanding charac- 
teristics of photomultipliers include 
a choice of photosensitive areas from 
a fraction of a square centimeter to 
hundreds of square centimeters, in- 
ternal amplifications ranging from 
10 to as much as 10 9 in some types, 
extremely fast time response with 
rise times as short as a fraction of 
a nanosecond, and output signal 
levels that are compatible with auxili- 
ary electronic equipment without 
need for additional signal amplifica- 
tion. 

Photomultipliers are used with 
scintillators and Cerenkov radiators 
for the detection and measurement 
of X-rays, gamma rays, and energetic 
particles, and find wide usage in nu- 
clear research, industrial controls, 
and space exploration. The availabil- 
ity of many different types of lasers 
has opened up many new fields of 
application for photomultipliers in 



Raman spectroscopy, laser ranging, 
precision measurements, and commu- 
nications. Photomultipliers are used 
in video playback equipment for 
home entertainment, and in the 
audio-visual educational field; optical 
character-recognition equipment util- 
izes photomultipliers for computer 
processing of financial statements. 

The history of the development of 
the photomultiplier is an interesting 
one. The use of secondary emission 
as a means for signal amplification 
was proposed as early as 1919. 1 The 
active development of photomulti- 
pliers was begun approximately 35 
years ago by combining a photo- 
emitter and one or more stages of 
secondary-electron-emission amplifi- 
cation in one vacuum envelope. In 
1935, lams and Salzberg 2 of RCA 
described a single-stage photomulti- 
plier which had a secondary-emission 
amplification of 6; in 1940, Rajch- 
man and Snyder 3 of RCA described 
a 9-stage electrostatically focused 
photomultiplier that served as a pro- 
totype design for a family of types 
that are still used throughout the 
world today. Other forms of photo- 
multipliers were developed in the 
late 1930's, and with the sharp in- 
crease in research in atomic energy 
and nuclear physics, much progress 
in photomultiplier development was 
made in the 1940's and 1950's. The 
rapid growth of electro-optics in the 
1960's further advanced the develop- 
ment of photomultipliers. The ad- 



RCA Photomultiplier Manual 



vent of the bialkali photocathode 
and more recently the Extended Red 
Multi- Alkali (ERMA) photocathode 
provided further improvements. Very 
recently, advances in solid-state 
theory and semiconductor-materials 
research have opened new vistas in 
the development of photomultipliers 
having performance capabilities that 
were literally impossible only a few 
years ago. 

REFERENCES 

1. J. Slepian, U. S. Patent 1,450.- 
265, April 3, 1923 (Filed 1919). 

2. H. lams and B. Salzberg, "The 
Secondary-Emission Phototube," 
Proc. IRE, Vol. 23, pp. 55-64 
(1935). 

3. J. A. Rajchman and R. L. 
Snyder, "An Electrostatically- 
Focused Multiplier Phototube," 
Electronics, Vol. 13, p. 20 
(1940). 

4. P. T. Farnsworth, "An Electron 
Multiplier," Electronics, Vol. 7, 
pp. 242-243 (1934). 

5. V. Zworykin, G. Morton, and 
L. Malter, "The Secondary- 
Emission Multiplier — A New 
Electronic Device," Proc. IRE, 
Vol. 24, pp. 351-375 (1936). 



6. G. Weiss, "On Secondary-Elec- 
tron Multipliers," Z. /. Techn. 
Physik, Vol. 17, No. 12, pp. 
623-629 (1936). 

7. W. Kluge, O. Beyer, and H. 
Steyskal, "Photocells with Sec- 
ondary-Emission Amplification," 
Z. f. Techn. Physik, Vol. 18, No. 
8, pp. 219-228 (1937). 

8. J. R. Pierce, "Electron-Multiplier 
Design," Bell Lab. Record, Vol. 
16, pp. 305-309 (1938). 

9. V. K. Zworykin and J. A. Rajch- 
man, "The Electrostatic Electron 
Multiplier," Proc. IRE, Vol. 27, 
pp. 558-566 (1939). 

10. C. C. Larson and H. Salinger, 
"Photocell Multiplier Tubes," 
Rev. Sci. Instr., Vol. 11, pp. 
226-229 (1940). 

11. R. C. Winans and J. R. Pierce, 
"Operation of Electrostatic 
Photomultipliers," Rev. Sci. 
Instr., Vol. 12, pp. 269-277 
(1941). 

12. R. W. Engstrom, "Multiplier 
Phototube Characteristics; Ap- 
plication to Low Light Levels," 
/. O. S. A., Vol. 37, No. 6, pp. 
421-431 (1947). 



Photoemission and Secondary 

Emission 



THIS section covers the means by 
which radiant energy is con- 
verted into electrical energy in a 
photomultiplier tube. 

Photomultipliers convert incident 
radiation in the visible, infrared, and 
ultraviolet regions into electrical sig- 
nals by use of the phenomenon of 



photoemission and then amplify the 
signals by means of secondary emis- 
sion. 

A basic component arrangement of 
a typical modern photomultiplier is 
shown in Fig. 1. Radiant energy en- 
ters the evacuated enclosure through 
a "window" which has a semitrans- 



IDENT RADIATION 



PHOTOCATHODE 
TO DYNODE No. I 
ELECTRON 
OPTICS 




SEMITRANSPARENT 
PHOTOCATHODE 



TYPICAL 

PHOTOELECTRON 

TRAJECTORIES 



VACUUM 
-ENCLOSURE 



-IZ:DYNODES 14: FOCUSING ELECTRODES 
I3:AN0DE 15: PHOTOCATHODE 



Fig. 1 — Schematic of typical photomultiplier showing some electron trajectories. 



RCA Photomultiplier Manual 



parent photocathode deposited upon 
its inner surface. The photocathode 
emits photoelectrons by the process 
of photoemission, i.e., by the inter- 
action of the incident radiant energy 
with electrons in the photocathode 
material. Photoelectrons from all 
parts of the photocathode are ac- 
celerated by an electric field so as to 
strike a small area on the first dy- 
node. Secondary electrons resulting 
from the process of secondary emis- 
sion (i.e., from the impact of the 
photoelectrons on the first dynode) 
are accelerated toward the second 
dynode by an electric field between 
dynodes No. 1 and No. 2 and im- 
pinge on the second dynode. The 
impact of the secondary electrons on 
dynode No. 2 results in the release 
of more secondary electrons which 
are then accelerated toward dynode 
No. 3. This process is repeated un- 
til the electrons leaving the last dy- 
node are collected by the anode and 
leave the photomultiplier as the out- 
put signal. If, on the average, 4 sec- 
ondary electrons are liberated at each 
dynode for each electron striking that 
dynode, the current amplification for 
a 12-stage photomultiplier is 4 12 , or 
approximately 17 million. Thus, the 
liberation of a single photoelectron 
at the photocathode results in 17 
million electrons being collected at 
the anode. Because this pulse has a 
time duration of approximately 5 
nanoseconds, the anode current is 
approximately 1 milliampere at the 
peak of the anode-current pulse. 

PHOTOEMISSION AND 
PHOTOCATHODES 

Photoemission is a process in 
which electrons are liberated from 
the surface of a material by the inter- 
action of photons of radiant energy 
with the material. 1 The energy of a 
photon E p is given by 



hv 



he 
K 



(1) 



where v is the frequency of the in- 
cident radiation, X is the wavelength 
of the incident radiation, h is Planck's 
quantum of action, and c is the 
velocity of light. Substitution of the 
numerical values of these constants 
from Appendix A provides the fol- 
lowing relation: 



1239.5 



(2) 



where E p is in electron-volts and X 
is in nanometers. Thus, photons of 
visible light which are in the wave- 
length range from 400 to 700 nano- 
meters have energies ranging from 
3.1 to 1.8 electron-volts. 

Fundamentals of Photoemission 

An ideal photocathode has a 
quantum efficiency of 100 per cent; 
i.e., every incident photon releases 
one photoelectron from the material 
into the vacuum. All practical photo- 
emitters have quantum efficiences be- 
low 100 per cent. To obtain a 
qualitative understanding of the vari- 
ations in quantum efficiency for 
different materials and for different 
wavelengths or photon energies, it is 
useful to consider photoemission as 
a process involving three steps: (1) 
absorption of a photon resulting in 
the transfer of energy from photon 
to electron, (2) motion of the elec- 
tron toward the material-vacuum 
interface, and (3) escape of the elec- 
tron over the potential barrier at 
the surface into the vacuum. 

Energy losses occur in each of 
these steps. In the first step, only 
the absorbed portion of the incident 
light is effective and thus losses by 
transmission and reflection reduce 
the quantum efficiency. In the sec- 
ond step, the photoelectrons may 



Photoemission and Secondary Emission 



lose energy by collision with other 
electrons (electron scattering) or 
with the lattice (phonon scattering). 
Finally, the potential barrier at the 
surface (called the work function in 
metals) prevents the escape of some 
electrons. 

The energy losses described vary 
from material to material, but a 
major difference between metallic 
and semiconducting materials makes 
separate consideration of each of 
these two groups useful. In metals, a 
large fraction of the incident visible 
light is reflected and thus lost to 
the photoemission process. Further 
losses occur as the photoelectrons 
rapidly lose energy in collisions with 
the large number of free electrons 
in the metal through electron-electron 
scattering. As a result, the escape 
depth, the distance from the surface 
from which electrons can reach the 
surface with sufficient energy to over- 
come the surface barrier, is small, 
and is typically a few nanometers. 
Finally, the work function of most 
metals is greater than three electron- 
volts, so that visible photons which 
have energies less than three electron- 
volts are prevented from producing 
electron emission. Only a few metals, 
particularly the alkali ones, have 
work-function values low enough to 
make them sensitive to visible light. 
Because of the large energy losses 
in absorption of the photon and in 
the motion of the photoelectron 
toward the vacuum (the first and 
second steps described above), even 
the alkali metals exhibit very low 
quantum efficiency in the visible re- 
gion, usually below 0.1 per cent (one 
electron per 1000 incident photons). 
As expected, higher quantum effi- 
ciencies are obtained with higher 
photon energies. For 1 2-electron-volt 
photons, quantum yields as high as 
10 per cent have been reported. 
The concept of the energy-band 



models that describe semiconductor 
photoemitters is illustrated in its 
simplest form in Fig. 2. Electrons 
can have energy values only within 
well defined energy bands which are 
separated by forbidden-band gaps. 

E fl - ELECTRON AFFINITY 



Eg-BAND GAP 



T 



CONDUCTION BAND E A 



FORBIDDEN BAND Eq 



Ea + Eq 



VALENCE BAND 



W\ 



■ SEMICONDUCTOR 



FERMI 
LEVEL 



VACUUM 



I 



Fig. 2 — Simplified semiconductor 
energy-band model. 

At 0°K, the electrons of highest en- 
ergy are in the so-called valence 
band and are separated from the 
empty conduction band by the band- 
gap energy E G . The probability that 
a given energy level may be occu- 
pied by an electron is described by 
Fermi-Dirac statistics and depends 
primarily on the difference in energy 
between the level under consideration 
and a reference level called the 
"Fermi level." As a first approxima- 
tion, it may be said that any energy 
levels which are below the Fermi 
level will be filled with electrons, and 
any levels which are above the Fermi 
level will be empty. At temperatures 
higher than 0°K, some electrons in 
the valence band have sufficient en- 
ergy to be raised to the conduction 
band, and these electrons, as well as 
the holes in the valence band created 
by the loss of electrons, produce 
electrical conductivity. Because the 
number of electrons raised to the 
conduction band increases with tem- 
perature, the conductivity of semi- 
conductors also increases with tem- 
perature. Light can be absorbed by 
valence-band electrons only if the 
photon energy of the light is at least 



8 



RCA Photomultiplier Manual 



equal to the band-gap energy E G . 
If, as a result of light absorption, 
electrons are raised from the valence 
band into the conduction band, 
photoconductivity is achieved. For 
photoemission, an electron in the 
conduction band must have energy 
greater than the electron affinity E A . 
The additional energy E A is needed 
to overcome the forces that bind the 
electron to the solid, or, in other 
words, to convert a "free" electron 
within the material into a free elec- 
tron in the vacuum. Thus, in terms 
of the model of Fig. 2, radiant energy 
can convert an electron into an in- 
ternal photoelectron (photoconduc- 
tivity) if the photon energy exceeds 
E G and into an external photoelectron 
(photoemission) if the photon energy 
exceeds (E G + E A ). As a result, 
photons with total energies E p less 
than (E G + E A ) cannot produce 
photoemission. 

The following statements can 
therefore be made concerning photo- 
emission in semiconductors. First, 
light absorption is efficient if the 
photon energy exceeds E G . Second, 
energy loss by electron-electron scat- 
tering is low because very few free 
electrons are present; thus, energy 
loss by phonon scattering is the pre- 
dominant loss mechanism. The es- 
cape depth in semiconductors is 
therefore much greater than in 
metals, typically of the order of tens 
of nanometers. Third, the threshold 
wavelength, which is determined by 
the work function in metals, is given 
by the value of (E G + E A ) in semi- 
conductors. Synthesis of materials 
with values of (E G + E A ) below 2 
electron-volts has demonstrated that 
threshold wavelengths longer than 
those of any metal can be obtained 
in a semiconductor. Semiconductors, 
therefore, are superior to metals in 
all three steps of the photoemissive 
process: they absorb a much higher 



fraction of the incident light, photo- 
electrons can escape from a greater 
distance from the vacuum interface, 
and the threshold wavelengths can 
be made longer than those of a metal. 
Thus, it is not surprising that all 
photoemitters of practical importance 
are semiconducting materials. 

In recent years, remarkable im- 
provements in the photoemission 
from semiconductors have been ob- 
tained through deliberate modifica- 
tion of the energy-band structure. 
The approach has been to reduce 
the electron affinity, E A , and thus to 
permit the escape of electrons which 
have been excited into the conduc- 
tion band at greater depths within 
the material. Indeed, if the electron 
affinity is made less than zero (the 
vacuum level lower than the bottom 
of the conduction band, a condition 
described as "negative electron affin- 
ity" and illustrated in Fig. 3), the 
escape depth may be as much as 
100 times greater than for the 
normal material. The escape depth 
of a photoelectron is limited by the 
energy loss suffered in phonon scat- 
tering. Within a certain period of 
time, of the order of 10~ 12 second, 
the electron energy drops from a 
level above the vacuum level to the 
bottom of the conduction band from 
which it is not able to escape into 
the vacuum. On the other hand, the 
electron can stay in the conduction 




EMPTY 

ACCEPTOR 

LEVELS FILLED 

ACCEPTOR 
LEVELS 



Fig. 3 — Semiconductor energy-band 

model showing negative electron 

affinity. 



Photoemission and Secondary Emission 



band in the order of 10 -10 second 
without further loss of energy, i.e., 
without dropping into the valence 
band. If the vacuum level is below 
the bottom of the conduction band, 
the electron will be in an energy state 
from which it can escape into the 
vacuum for a period of time that is 
approximately 100 times longer than 
if an energy above the bottom of 
the conduction band is required for 
escape, as in the materials repre- 
sented by Fig. 2. Therefore, a ma- 
terial conforming to the conditions 
of Fig. .3 has a greatly increased 
escape depth. Under such circum- 
stances, the photosensitivity is sig- 
nificantly enhanced. Substantial re- 
sponse is observed even for photons 
with energies close to that of the 
band gap where the absorption is 
weak. Efficient photoemission in this 
case results only because of the 
greater escape depth. 

The reduction of the electron affin- 
ity is accomplished through two 
steps. First, the semiconductor is 
made strongly p-type by the addition 
of the proper "doping" agent. For 
example, if gallium arsenide is the 
host material, zinc may be incor- 
porated into the crystal lattice to a 
concentration of perhaps 1 ,000 parts 
per million. The zinc produces iso- 
lated energy states within the for- 
bidden gap, near the top of the 
valence band, which are normally 
empty, but which will accept elec- 
trons under the proper circumstances. 
The p-doped material has its Fermi 
level near the top of the valence 
band. The second step is to apply 
to a semiconductor a surface film 
of an electropositive material such 
as cesium. Each cesium atom be- 
comes ionized through loss of an 
electron to a p-type energy level 
near the surface of the semiconduc- 
tor, and is held to the surface by 
electrostatic attraction. 



The changes which result in the 
energy-band structure are two-fold. 
In the first place, the acceptance of 
electrons by the p-type impurity 
levels is accompanied by a down- 
ward bending of the energy bands. 
This bending can be understood by 
observing that a filled state must be, 
in general, below the Fermi level; 
the whole structure near the surface 
is bent downward to accomplish this 
result. In the second place, the 
potential difference between the 
charged electropositive layer (cesium) 
and the body charge (filled zinc 
levels) results in a further depres- 
sion of the vacuum level as a result 
of a dipole moment right at the 
surface. 

Another way to describe the re- 
duction of the electron affinity is to 
consider the surface of the semicon- 
ductor as a capacitor. The charge on 
one side of the capacitor is repre- 
sented by the surface layer of cesium 
ions; the other charge is represented 
by the region of filled acceptor levels. 
The reduction in the electron affinity 
is exactly equal to the potential dif- 
ference developed across the ca- 
pacitor. 

In a more rigorous analysis, the 
amount by which the energy bands 
are bent is found to be approximately 
equal to the band-gap, and the 
vacuum level is lowered until the 
absorption level of the electroposi- 
tive material is essentially at the top 
of the valence band. 

Measurements of 
Photocathode Sensitivity 

There are various ways of specify- 
ing the sensitivity of a photocathode. 
For many purposes, a knowledge of 
the response of the photocathode to 
particular wavelengths or equivalent 
energies of radiation is necessary. 
Sometimes it is convenient to have 



10 



RCA Photomultiplier Manual 



a single number to describe the re- 
sponse of the photocathode to some 
standard broad-band source, such as 
a tungsten lamp. 

Variation in photocathode sensi- 
tivity over the range of wavelengths 
to which the photocathode material 
is sensitive is usually described by 
means of a spectral-response curve 
such as the one shown in Fig. 4. 
The abscissa may represent the wave- 
length in nanometers or angstroms, 
or the photon energy in electron- 
volts. The values can be converted 
from one scale to the other by use 
of Eq. 1. 

The ordinate can also be expressed 
in many ways. The most common 
scale is amperes per incident watt, 
as shown; a second common scale is 
that of quantum efficiency, the av- 
erage numbers of photoelectrons 
emitted per incident photon. 



no 






80 




-. 


60 


- 


/ \ 


40 


- 


/ Y*^K 2 CsSb 


20 


- 


\ 


10 
8 


- 


1 


6 


- 


1 


4 


- 




2 


1 1 1 


1 1 ill 1 



200 400 600 800 

WAVELENGTH — NANOMETERS 

1 — i 1 1 

8 6 4 2 

PHOTOELECTRON ENERGY— eV 

Fig. 4 — Typical spectral-response 

curve for a bialkali photocathode with 

a 008 lime-glass window. 



It should be noted that the spec- 
tral response of photoemitters is 
modified by the window material. 
Then sf ore, the short-wavelength limit 
of the spectral response is generally 
a characteristic of the window rather 
than of the photocathode. 

A common practice is to specify 
the response of the device to the in- 
cident flux from a tungsten lamp with 
a lime-glass window operating at a 
color temperature of 2854°K. The 
units of measurement are most fre- 
quently amperes per lumen. Some- 
times sensitivity is expressed in 
terms of photocurrent and the total 
incident radiant power from the 
lamp on the photocathode window. 
This sensitivity is commonly stated 
in amperes per watt. It is the general 
practice to give sensitivity measure- 
ments in terms of incident flux. For 
some purposes, such as the study of 
photoemission phenomena, it may 
be useful to specify the sensitivity in 
terms of absorbed flux. 

Opaque and Semitransparent 
Photocathodes 

The photocathode consists of a 
material that emits electrons when 
exposed to radiant energy. There 
are two types of photocathodes. In 
one, the opaque photocathode, the 
light is incident on a thick photo- 
emissive material and the electrons 
are emitted from the same side as 
that struck by the radiant energy. In 
the second type, the semitranspar- 
ent photocathode, the photoemissive 
material is deposited on a trans- 
parent medium so that the electrons 
are emitted from the side of the 
photocathode opposite the incident 
radiation. 

Because of the limited escape 
depth of photoelectrons, the thick- 
ness of the semitransparent photo- 
cathode film is critical. If the film 



Photoemission and Secondary Emission 



11 



is too thick, much of the incident 
radiant energy is absorbed at a dis- 
tance from the vacuum interface 
greater than the escape depth; if the 
film is too thin, much of the incident 
radiant energy is lost by transmis- 
sion. Because radiant-energy absorp- 
tion varies with wavelength, as shown 
in Fig. 5, the spectral response of a 
semitransparent photocathode can be 
controlled to some extent by control 
of its thickness. With increasing 
thickness, for example for alkali 
antimonides, blue response decreases 
and red response increases because of 
the difference in absorption coeffi- 
cients. 




400 800 I200 I60O 

WAVELENGTH — NANOMETERS 

Fig. 5 — Radiation absorption for three 
commonly used semitransparent photo- 
cathode materials. Note loss of absorp- 
tion in red region. 

Photocathodes of 
Practical Importance 

The photocathodes most com- 
monly used in photomultipliers 
are silver-oxygen-cesium (Ag-O-Cs), 
cesium-antimony (Cs 3 Sb), multi- 
alkali or trialkali [(Cs) Na 2 KSb]*, 

* The parenthetical expression (Cs) in- 
dicates trace quantities of the element. 



and bialkali (K 2 CsSb). Typical spec- 
tral-response curves for these ma- 
terials are shown in Fig. 4 and Fig. 
6. Additional information about 
these and other photocathodes of 
practical importance is shown in 
Table I. 

The long-wavelength response of 
the multialkali photocathode has 
been extended recently by process- 
ing changes including the use of an 
increased photocathode-film thick- 
ness at the expense of the short- 
wavelength response. A typical 
spectral-response curve for one of 
these Extended Red Multi-Alkali 
(ERMA) photocathodes is shown in 
Fig. 7 in comparison with a con- 
ventionally processed multialkali 
photocathode. 



100 
s 

6 

»- " 

1- 

< 

s 

I 2 

I 

>- 





y^ 




-/ 




(Cs)Na 2 KSb 0/<> 






-X— Cs 3 Sb 


-_ / 




1 J^ 


-/ 




I r- Ag-O-Cs 


- A 




^4L^-0A°^ 


z / 






i i 


i i i 


1 III 1 \ 



200 400 600 800 1000 
WAVELENGTH — NANOMETERS 

Fig. 6 — Typical spectral-response 
curves, with 008 lime-glass window, 
for (a) silver-oxygen-cesium (Ag-O-Cs), 
(b) cesium-antimony (Cs,Sb), (c) multi- 
alkali or trialkali [(Cs)Na l KSb]. 



12 RCA Photomultiplier Manual 

Table I — Nominal Composition and Characteristics of Various 
Photocathodes. 















WavekngUi 
















Conversion 




ol 




Quantum 


Dark 


Nominal 


Response 


Typo of 


Envelope 


Factor b 


Luminous 


Maximum 


Sensitivity 


Efficiency 


Emission 


Composition 


Desifnaaon 


Photo- 
cathodi 


Material^ 


(lumen/watt 
at Xmax) 


Sensitivity 
OiA/lumin) 


so 


at Xmax 
(mA/watt) 


at Xmax 
(percent) 


It zs°c 

Axl0-»/em 2 


Ag-O-Cs 


S-l 





0080 


92.7 


25 


800 


2.3 


0.36 


900 


Ag-O-Rb 


S-3 





0080 


285 


6.5 


420 


1.8 


0.55 


— 


Cs3Sb 


S-19 





Si02 


1603 


40 


330 


64 


24 


0.3 


Cs3Sb 


S-4 





0080 


1044 


40 


400 


42 


13 


0.2 


Ca3Sb 


S-5 





9741 


1262 


40 


340 


50 


18 


0.3 


CS3B1 


S-8 





0080 


757 


3 


365 


2.3 


0.77 


0.13 


Ag-Bi-0-Cs 


S-10 


S 


0080 


509 


40 


450 


20 


5.6 


70 


Cs3Sb 


S-13 


S 


Si02 


799 


60 


440 


48 


14 


4 


Cs3Sb 


S-9 


S 


0080 


683 


30 


480 


20 


5.3 


— 


Cs3Sb 


S-ll 


S 


0080 


808 


60 


440 


48 


14 


3 


Cs3Sb 


S-21 


S 


9741 


783 


30 


440 


23 


6.7 


— 


CsjSb 


S-17 


O 


0080 


667 


125 


490 


83 


21 


1.2 


Na 2 KSb 


S-24 


S 


7056 


1505 


32 


380 


64 


23 


0.0003 


K-Cs-Sb 





S 


7740 


1117 


80 


400 


89 


28 


0.02 


(Ca)Na2KSb 


„ 


S 


Si02 


429 


150 


420 


64 


18.9 


0.4 


(Cs)Na2KSb 


S-20 


s 


0080 


428 


150 


420 


64 


19 


0.3 


(Cs)Na2KSb 


S-25 


s 


0080 


276 


160 


420 


44 


13 


— 


(Cs)Na2KSb 


ERMA C 


s 


7056 


169 


265 


575 


45 


10 


1 


Ga-As 


__ 


h 


9741 


148 


250 


450 


37 


10 


0.1 


Ga-As-P 


— 


0> 


Sapphire 310 


200 


450 


61 


17 


0.01 


lnGaAs-CsO d 


_ 


h 


0080 


266 


260 


400 


71 


22 


l d 


Cs2Te 


_ 


s 


LiF 


" 


e 


120 


12.6 


13 


f 


Csl 





s 


LiF 


e 


« 


120 


24 


20 


f 


Cul 


— 


s 


LiF 


e 


e 


150 


13 


10.7 


f 



Glass 



» Numbers refer to the following glasses: 
0080— Corning Lime Glass 
9741— Corning Ultraviolet Transmitting 
7056— Corning Borosilicate Glass 
7740— Corning Pyrex Glass ,.,,,,,„ 

Sifts— Fused Silica (Suprasil— Trademark of Engle- 
hard Industries, Inc., Hillside, New Jersey) 

"These conversion factors are the ratio of the 
radiant sensitivity at the peak of the spectral 
response characteristic in amperes per watt to 
the luminous sensitivity in amperes per lumen for 
a tungsten test lamp operated at a color tempera- 
ture of 2854°K. 
c An RCA designation for "Extended-Red Multialkali." 
a An experimental photocathode, private communi- 
cation from B. F. Williams, RCA Laboratories, 



Princeton, New Jersey. The dark emission indi- 
cated is a calculated value based on a bandgap 
of 1.1 eV for the particular composition studied. 
See also B. F. Williams, "InGaAs-CsO, A Low 
Work Function (Less Than 1.0 eV) Photoemitter, 
Appl. Phys. Letters 14, No. 9, p. 273 (1969). 

• Not relevant. 

' Data unavailable; expected to be very low. 

e Reflecting substrate. 

h Single crystal. 

1 Polycrystalline. 

O = Opaque 

S = Semitransparent 



Photoemission and Secondary Emission 



13 



100- 



6 




N, 



° =TT O) 



400 600 800 1000 

WAVELENGTH - NANOMETERS 

Fig. 7 — Typical spectral-response 
curve for one of the Extended Red 
Multi-Alkali (ERMA) photocathodes 
in comparison with a conventionally 
processed multialkali photocathode. 

New Photocathode Materials 

The negative-electron-affinity ma- 
terials described earlier are used in 
ipaque photocathodes. Typical spec- 
t "al-response curves for GaAs(Cs) 
aid GaAs ]! P 1 _ I (Cs) opaque photo- 
cithodes are shown in Fig. 8. De- 
velopment of negative-electron-affin- 
ity materials is proceeding rapidly. 
Therefore, improved sensitivities and 
extended spectral ranges may be an- 
ticipated. 

SECONDARY EMISSION 

When electrons strike the surface 
of a material with sufficient kinetic 
energy, secondary electrons are 
emitted. The secondary-emission 
ratio or yield, 8, is defined as follows: 



where N s is the average number of 
secondary electrons emitted for N e 
primary electrons incident upon the 
surface. 



5 



100 

8 

6 






4 




/I /V*- 'GaAsP 






/ /^ \ \^# — Go As 


2 






10 
8 




1 1 J& 


6 


- 




4 


- 






2 


_ 


/ 






1 


/ | 


1 1 1 1 1 


1 1 



200 400 600 800 1000 
WAVELENGTH — NANOMETERS 

Fig. 8 — Typical spectral-response 

curves for GaAs(Cs) and GaAs,Pi-,(Cs) 

opaque photocathodes. 

Fundamentals of 
Secondary Emission 



The physical processes involved 
in secondary emission are in many 
respects similar to those already de- 
scribed under Fundamentals of 
Photoemission. The main difference 
is that the impact of primary elec- 
trons rather than incident photons 
causes the emission of electrons. The 
steps involved in secondary emission 
can be stated briefly as follows: 



14 

1. The incident electrons interact 
with electrons in the material and 
excite them to higher energy states. 

2. Some of these excited electrons 
move toward the vacuum-solid inter- 
face. 

3. Those electrons which arrive at 
the surface with energy greater than 
that represented by the surface bar- 
rier are emitted into the vacuum. 

When a primary beam of electrons 
impacts a secondary-emitting ma- 
terial, the primary-beam energy is 
dissipated within the material and a 
number of excited electrons are pro- 
duced within the material. Based on 
experimental data, approximations 
have been made 2 based on the as- 
sumption that the range of primary 
electrons varies as the 1.35 power of 
the primary energy and that the num- 
ber of electrons excited is uniform 
throughout the primary range. The 
numbers of excited electrons pro- 
duced are indicated in Fig. 9 for 
primary energies varying from 400 



RCA Photomultiplier Manual 

to 2200 electron-volts. The total 
number of excited electrons pro- 
duced by a primary is indicated by 
the area of the individual rectangles 
in the figure. 

As an excited electron in the bulk 
of the material moves toward the 
vacuum-solid interface, it loses en- 
ergy as a result of collisions with 
other electrons and optical phonons. 
The energy of the electron is very 
rapidly dissipated as a result of 
these collisions, and it is estimated 
that the energy of such an electron 
will decay to within a few times the 
mean thermal energy above the bot- 
tom of the conduction band within 
10- 12 second. If the electron ar- 
rives at the vacuum-solid interface 
with energy below that required to 
traverse the potential barrier, it can- 
not escape as a secondary electron. 
Therefore, only those electrons ex- 
cited near the surface of the ma- 
terial are likely to escape as second- 
ary electrons. The probability of 




20 



40 60 80 

DEPTH — NANOMETERS 



100 



Fig. 



9 — An illustration showing the processes of secondary emission, 
for explanation. 



See text 



Photoemission and Secondary Emission 



15 



escape for an excited electron is 
assumed to vary exponentially with 
the excitation depth, as indicated in 
Fig. 9. If the product of the escape 
function and the number of excited 
electrons (which is a function of 
primary energy and depth) is inte- 
grated, a secondary-emission-yield 
function may be obtained, as indi- 
cated in the insert at the top of Fig. 
9. The model which has been as- 
sumed thus explains the general 
characteristics of secondary emis- 
sion as a function of primary energy. 
Secondary-emission yield increases 
with primary energy, provided the 
excited electrons are produced near 
the surface where the escape prob- 
ability is high. As the primary- 
electron energy increases, the number 
of excited electrons also increases, 
but the excitation occurs at greater 
depths in the material where escape 
is much less probable. Consequently, 
the secondary-emission yield even- 
tually reaches a maximum and then 
decreases with primary energy. 



Experimental secondary-emission- 
yield values are shown as a func- 
tion of primary-electron energy in 
Fig. 10 for MgO, a traditional sec- 
ondary-emission material, and for 
GaP(Cs), a recently developed nega- 
tive-electron-affinity material. Al- 
though both of these materials display 
the general characteristics of sec- 
ondary emission as a function of 
primary energy, as expected from 
the model illustrated in Fig. 8, the 
secondary-emission yield for GaP(Cs) 
increases with voltage to much 
higher values. Even though electrons 
are excited rather deep in the nega- 
tive-electron-affinity material and 
lose most of their excess energy as 
a result of collisions, many still es- 
cape into the vacuum because of 
the nature of the surface barrier. 

When secondary electrons are 
emitted into the vacuum, the spread 
of emission energies may be quite 
large, as illustrated in the curve of 
Fig. 11 for a positive-affinity emit- 
ter. The peak at the right of the 



300 



i i r 



i i i 



i i i i i i i i 




i i i i i i iiii 



20 



• Typical curve of secondary-emission yield as a function of primary- 
electron energy in GaP(Cs) and MgO. 



16 



RCA Photomultiplier Manual 



S ioo 




20 



40 60 80 
ELECTRON" ENERGY - 



IOO I20 
ELECTRON VOLTS 



Fig. 11 — Typical secondary-electron energy distribution; peak at right is caused 
by reflected primary electrons. 



curve does not represent a true sec- 
ondary, but rather a reflected pri- 
mary. Data are not available for the 
emission energies from a negative- 
electron-affinity material, but they 
are expected to be considerably less 
than for positive-affinity materials. 

Negative electron affinity has not 
only expanded the field of photo- 
emission, but has proved very im- 
portant in the development of greatly 
improved secondary-emitting ma- 
terials for use as dynodes in photo- 
multipliers. With negative-electron- 
affinity materials, such as GaP, the 
secondary-emission ratio increases 
almost linearly up to very high pri- 
mary energies; factors up to 130 
have been measured. This factor 
compares with typical values of less 
than 10 for previously used dynode 
materials. 

Effects of 
Secondary-Emission Statistics 

The statistical distribution of sec- 
ondary-emission yields tends to de- 



grade the signal-to-noise ratio in a 
photomultiplier; the greatest contri- 
bution to the noise occurs at the first 
dynode. If the secondary-emission 
ratio of the first dynode is improved 
through the use of a negative-elec- 
tron-affinity material, such as GaP, 
the emission statistics of the first dy- 
node are markedly improved and the 
performance of the multiplier more 
closely resembles that of a perfect 
amplifier. Photomultiplier statistics 
are explained in greater detail in the 
section on Statistical Fluctuation and 
Noise. 

The incorporation of high-gain sec- 
ondary-emission materials through- 
out the dynode chain makes it 
possible to design photomultipliers 
with fewer stages for given amplifi- 
cations. The use of fewer stages also 
reduces the variation of gain with 
voltage changes. 

Time Lag in Photoemission 
and Secondary Emission 

Because both photoemission and 
secondary emission can be described 



Photoemission and Secondary Emission 



17 



in terms of the excitation of elec- 
trons within the volume of the solid 
and the subsequent diffusion of these 
electrons to the surface, a finite time 
interval occurs between the instant 
that a primary (photon or electron) 
strikes a surface and the emergence 
of electrons from the surface. 
Furthermore, in the case of second- 
ary emission, the secondaries can be 
expected to reach the surface over a 
period of time. Within the limita- 
tions of a mechanistic approach to 
a quantum phenomenon, time inter- 
vals for metals or insulators of the 
order of 10 -13 to 10~ 14 second may 
be estimated from the known energy 
of the primaries, their approximately 
known range, and the approximately 
known diffusion velocities of the in- 
ternal electrons. In negative-electron- 
affinity semiconductors, it is known 
that the lifetime of internal "free" 
electrons having quasi-thermal ener- 
gies (i.e. electrons near the bottom 
of the conduction band) can be of 
the order of 10~ 10 second. 

Thus far experiments have pro- 
vided only upper limits for the time 
lag of emission. In the case of sec- 
ondary emission, a variety of ex- 
periments have established limits. 
Several investigators 3 > 4 > 6 have de- 
duced limits from the measured per- 
formance of electron tubes using 
secondary emitters. Others, making 
direct measurements of these limits, 
determined the time dispersion of 
secondary emission by letting short 
electron bunches strike a target and 
comparing the duration of the result- 
ing secondary bunches with the 
measured duration of the primary 
bunch. By this means an upper limit 
of 6 x 10 -12 second was determined 
for platinum 7 and an upper limit 
of 7 X 10- u second for an MgO 
layer B formed on the surface of an 
AgMg alloy. 

The upper limit for the time lag 



in photoemission, however, is not 
well established. One investigation 8 
determined a limit of 3 X 10 -9 sec- 
ond. From careful measurements 9 
of the time performance of fast 
photomultipliers it can be inferred 
that the limit must be at least an 
order of magnitude smaller. 

While these limits are a useful 
guide to the type of time perform- 
ance to be expected in present 
photomultipliers, they will probably 
have less significance as new photo- 
multipliers using semiconducting 
photoemitters and secondary emit- 
ters are developed. Semiconductors 
having minority-carrier lifetimes of 
the order of microseconds are now 
available. Probably, by combination 
of this characteristic with negative 
electron affinity, higher gains and 
quantum efficiencies can be achieved, 
but at a sacrifice of time response or 
band width. However, the first gen- 
eration of negative-electron-affinity 
emitters (e.g., GaP) has actually re- 
sulted in photomultipliers having 
better time performance because a 
smaller number of stages operating 
at higher voltage can be used. At 
this time it can only be concluded 
that in the future photomultipliers 
will probably be designed to match 
in more detail the requirements of 
a particular use. 



REFERENCES 

1. A. H. Sommer, Photoemissive 
Materials, John Wiley, New York 
(1968) 

2. R. E. Simon and B. F. Williams, 
"Secondary Electron Emission," 
IEEE Trans. Nucl. Sci., Vol. NS- 
15, No. 3, p. 167 (1968) 

3. G. Diemer and J. L. H. Jonker, 
"On the Time Delay of Second- 
ary Emission," Philips Research 
Repts., Vol. 5, p. 161 (1950) 



18 



RCA Photomultiplier Manual 



4. M. H. Greenblatt and P. A. 
Miller, Jr., "A Microwave Second- 
ary-Electron Multiplier," Phys. 
Rev., Vol. 72, p. 160 (1947) 

5. M. H. Greenblatt, "On the Mea- 
surement of the Average Time 
Delay in Secondary Emission," 
RCA Rev., Vol. 16, p. 52 (1955) 

6. C. G. Wang, "Reflex Oscillators 
Using Secondary-Emission Cur- 
rent," Phys. Rev., Vol. 68, p. 284 
(1945) 



7. E. W. Ernst and H. Von Foerster, 
"Time Dispersion of Secondary 
Electron Emission," /. Appl. 
Phys., Vol. 26, p. 781 (1955) 

8. E. O. Lawrence and J. W. Beams, 
"Instantansity of the Photoelectric 
Effect," Phys. Rev., Vol. 29, p. 
903 (1927) 

9. M. Birk, Q. A. Kerns, and R. F. 
Tusting, "Evaluation of the 
C-70045A High-Speed Photomul- 
tiplier," IEEE Trans. Nucl. Sci. 
Vol. NS-11, p. 129 (1964) 



19 



Principles of Photomultiplier 

Design 



THIS section describes the physi- 
cal and electrical parameters of 
interest in the design of photomul- 
tipliers. The effects of dynode and 
anode structures on such parameters 
as time response, sensitivity, and dark 
current are discussed. A section cov- 
ering ruggedized photomultipliers de- 
scribes their structure and method of 
testing. 

PHOTOCATHODE-TO-FIRST 
DYNODE REGION 

As mentioned in the preceding 
section, two classes of photocathodes 
are used in photomultipliers: opaque 
and semitransparent. Because opaque 
photocathodes are an integral part 
of the electron-multiplier structure, 
they are discussed later in connec- 
tion with the electron multiplier. 
Semitransparent photocathodes are 
formed on planar or spherical-section 
windows which are part of the photo- 
cathode-to-first dynode region. Semi- 
transparent photocathodes have large, 
readily accessible light-collection 
areas which may be directly coupled 
to scintillation crystals. 

A planar-photocathode design, 
such as that shown in Fig. 12(a), 
provides excellent coupling to a scin- 
tillation crystal, but its time re- 
sponse is not as good as that of a 
spherical-section photocathode. The 
spherical-section photocathode shown 
in Fig. 12(b) when coupled to a 



high-speed electron multiplier pro- 
vides a photomultiplier having a 
very fast time response. Some tube 
types utilize a spherical-section 
photocathode on a plano-concave 
faceplate to facilitate scintillator 
coupling. This design is usually 
limited to faceplate diameters of two 
inches or less because of the exces- 
sive thickness of the glass at the edge. 
The plano-concave faceplate may 
also contribute to some loss in uni- 
formity of sensitivity because of in- 
ternal reflection effects near the thick 
edge of the photocathode. 

Design Analysis 

Photocathode-to-first-dynode struc- 
tures are usually axially symmetric, 
with the symmetry axis passing 
through the center of the photo- 
cathode. This symmetry simplifies 
the electron-optical analysis of the 
photocathode-to-first-dynode region 
because the problem is reduced from 
three dimensions to two. The first 
step in the analysis is the solution of 
the Laplace equation, V 2 V = 0. 
Because a solution of this equation 
in closed form is impossible for most 
geometries, numerical solutions are 
obtained; 1 one solution 2 results in 
the photocathode-to-first-dynode re- 
gion shown in Fig. 12(b). Perform- 
ance is determined by use of a 
computer to trace equipotential lines 
and electron trajectories which can 



20 



RCA Photomultiplier Manual 




FOCUSING 
'ELECTRODE 



FACEPLATES- 



SEMI- 
TRANSPARENT 
PHOTOCATHODE 



INTERNAL 

CONDUCTIVE 

COATING 




FRONT-END REGION 

(a) 



FACEPLATE 




Fig. 12 ■ 



FRONT-END REGION 

(b) 

■Front-end region of a photomultiplier with (a) a planar photocathode, 
and (b) a spherical-section photocathode. 



then be superimposed on a schematic 
diagram of the tube structure as 
shown in Fig. 13. 

Collection efficiency and time re- 
sponse may be predicted from an 
analysis of the electron trajectory. 
The collection efficiency is defined 
as the ratio of the number of photo- 
electrons which land upon a useful 
area of the first dynode to the num- 
ber of photoelectrons emitted by the 
photocathode. If all the photoelec- 
trons began their trajectories at the 
surface of the photocathode with 
zero velocity, a collection efficiency 
of 100 per cent would be possible; 
however, because of the finite initial 



velocities, 3 some photoelectrons be- 
gin their trajectories with unfavorable 
angles of launch and are not col- 
lected on the useful area of the first 
dynode. Although collection effi- 
ciency is difficult to measure, it can 
be calculated by computer by the 
use of Monte Carlo simulations. 4 

In modern photomultiplier struc- 
tures, first-dynode collection efficien- 
cies vary from 85 to 98 per cent. 
Ideally, the emitted photoelectrons 
should converge to a very small area 
on the first dynode; in practice this 
electron-spot diameter is approxi- 
mately 1/8 to 1/20 of the cathode 
diameter, depending upon tube type. 



Principles of Photomultiplier Design 



21 



PHOTOCATHODE 




TYPICAL 

ELECTRON 

TRAJECTORIES 



TYPICAL 

EQUIPOTENTIAL 

LINES 



DYNODE No.l 



Fig. 13 — Cross section of a photomultiplier showing equipotential lines and 
electron trajectories that were plotted by computer. 

first dynode allows simple photoelec- 
ELECTRON MULTIPLIERS tron collector systems to be used. 

This structure is used only when 
time response is not of prime consid- 
eration. The relatively slow time re- 
sponse is a result of the weak elec- 
tric field at the surfaces of the dynode 
vanes. This structure is the most 
flexible as to the number of stages. 

The box-and-grid dynode struc- 
ture, shown in Fig. 16, also has a 
large first-dynode collection area. 
The time response of this structure 
is similar to that of a venetian-blind 
multiplier (slow) because of the weak 
electric field within the box. 

A linear-multiplier structure, such 
as that shown in Fig. 17, offers good 
time response. Because the dynode 
stages (with the exception of the 
first three and the last two) are itera- 
tive, the number of stages in this 
structure may be varied without elec- 
tron-optic redesign. 

The continuous-channel multiplier 
structure 5 shown in Fig. 18 is very 



In a photomultiplier having an 
opaque photocathode, the photo- 
cathode is part of the electron- 
multiplier or dynode-chain structure. 
The circular cage structure, shown 
in Fig. 14(a) for a "side-on" type of 
photomultiplier, employs an opaque 
photocathode and is one of the earli- 
est multiplier configurations. Its rela- 
tively fast time response is a result 
of its small size and high interdynode 
field strengths. This same structure 
is also used in "head-on" type photo- 
multipliers, as shown in Fig. 14(b). 
Because of the type of symmetry in 
the design, the number of dynodes 
in these structures can be reduced by 
units of two. 

The "venetian-blind" dynode struc- 
ture shown in Fig. 15 has a compact, 
rugged geometry and is generally 
used with planar photocathodes be- 
cause the relatively large area of the 



22 



RCA Photomultiplier Manual 




O = OPAQUE PHOTOCATHODE 
' -9 = DYNODE * ELECTRON 

MULTIPLIER 
10 = ANODE 



(a) 




1^. INCIDENT 
^ RADIATION 



••-FACEPLATE 



I -10: DYNODES= ELECTRON MULTIPLIER 
II : ANODE 



(b) 

Fig. 14 — The circular-cage multiplier structure (a) in a "side-on" photomultiplier, 
(b) in a "head-on" photomultiplier. 



ELECTRON MULTIPLIER 



S 






\ / \ v \ 



<•>'/' vi: 



SEMITRANSPARENT 
PHOTOCATHODE 



<" / /S 



,]/ \V'\, /,•>,/ 
d/'\'/-.\ / \,/ 



m 



FOCUSING 
-ELECTRODE 



INTERNAL 

CONDUCTIVE 

COATING 



IVE / Vjt. 



Fig. 15 — The venetian-blind multiplier structure. 



Principles of Photomultiplier Design 



23 



SEMITRANSPARENT 
PHOTOCATHODE - 




I-9-DYN0DES" ELECTRON MULTIPLIER 
IO-ANODE 



Fig. 16 — The box-and-grid multiplier structure. 



<- FOCUS RIN8 




SEMI- 
TRANSPARENT \\ 
PHOTOCATHOOE — -»A 



, I INCIDENT 
1 RADIATION 



FOCUSING 
ELECTRODE 



FACEPLATE- 



INTERNAL 
CONDUCTIVE .. 
COATIN6 II 

// 



A 



l-IO- DYNODES = ELECTRON MULTIPLIER 
ll'ANODE 



Fig. 17 — The linear-multiplier structure. 



compact and utilizes a resistive emit- 
ter on the inside surface of a cylinder 
rather than a discrete number of dy- 
nodes. The lack of discrete dynodes 
causes the electron-multiplication sta- 
tistics to be poor because of the vari- 
able path lengths and the variable 
associated voltages. The gain of a 
continuous-channel multiplier is de- 
termined by the ratio of length, 1, 
to inside diameter, d; a typical value 
of this ratio is 50 but it may range 
from 30 to 100. 

An interesting multiplier structure 
which has potentially good time re- 
sponse is a series of close-spaced 
parallel-plane structures that use 
transmission dynodes. The dynodes 
consist of thin membranes on which 



primary electrons impinge to cause 
secondary emission from the oppo- 
site side. There are, however, diffi- 
culties in this construction: (1) in 
obtaining rugged self-supporting dy- 
nodes of reasonable size, and (2) in 
preventing transmission of primary 
electrons which tend to degrade the 
statistics of electron transit time. 




Fig 18 — The continuous-channel 
multiplier structure. 



24 



RCA Photomultiplier Manual 



Multiplier Analysis 

The analysis of the multiplier 
structure requires solution of the 
Laplace equation. This solution can 
be obtained with an analog model, 
such as the rubber membrane, 6 or 
in digital form by use of a com- 
puter. 1 Initial analysis requires that 
electrons from a large area of one 
dynode be converged onto a smaller 
area on the next dynode. Converg- 
ence must take place in two dimen- 
sions to keep the electron cascade 
from spilling off the ends of the dy- 
nodes. The over-all rounded shape 
of the dynode shown in Fig. 19 con- 
verges the electrons in one dimen- 
sion, while the field-forming ridges 
at or near the dynode ends converge 
the electrons in the second dimen- 
sion. 

In an analysis of multiplier struc- 
tures, space-charge effects may be 
ignored in the early stages because 
of the low current densities. How- 
ever, space-charge effects must be 
taken into account in the output 



400 — 



< 300- 



200 



100 





-psHfcyjjjy 


: 








r=^k 




^ 


l T _... _ 
RIDGE ~f RIDGE -l 



Fig. 19 — A dynode whose over-all 

rounded shape and field-forming ridges 

converge the electron cascade. 

stages of tube designs requiring large 
anode pulse currents. Such tubes 
usually require higher interstage po- 
tential differences near the anode to 
overcome space-charge saturation 
effects. 

ANODES 

The primary function of the anode 
is to collect secondary electrons 
from the last dynode. The anode 
should exhibit a constant-current 
characteristic of the type shown in 
Fig. 20. The simplest anode struc- 
ture, shown in Fig. 21, is a grid-like 
collector used in some venetian-blind 
structures. The secondary electrons 
from the next-to-last dynode pass 



*» 



50 100 150 200 

POTENTIAL BETWEEN LAST DYNODE AND ANODE - 



250 



■VOLTS 



Fig. 20 — Constant-current characteristic required of an anode. 



Principles of Photomultiplier Design 



25 




LAST DYNODE 



/?,-g, 21 — The simplest anode structure, 
a grid-like collector. 

through the grid to the last dynode. 
Secondary electrons leaving the last 
dynode are then collected on the 
grid-like anode. Photomultipliers de- 
signed to have fast time responses 
require anodes with matched-imped- 
ance transmission lines. Most high- 
speed circuits are designed to utilize 
a 50-ohm impedance, which requires 
a 50-ohm transmission line within the 
tube and a suitable connector or 
lead geometry outside the tube to 
permit proper interface. A more de- 
tailed description of these require- 
ments is given in the section on 
Voltage-Divider Considerations. 

Anode configurations and internal 
transmission lines may be analyzed 
by use of the standard methods of 
cavity and transmission-line analy- 
sis. These methods yield approximate 
design parameters, 7 which are op- 
timized experimentally by means of 
time-domain reflectometry, TDR 8 - 9 . 
TDR provides information about the 
discontinuities in the characteristic 
impedance of a system as a function 
of electrical length and is an ex- 
tremely useful approach in the de- 
sign of voltage-divider circuits and 
mating sockets which do not readily 
lend themselves to mathematical 
analysis. 

Certain anode structures in fast- 
rise-time photomultipliers exhibit a 
small-amplitude pulse (prepulse) that 
can be observed a nanosecond or 
two before the true signal pulse. In 
grid-like anode structures this pre- 
pulse is induced when electrons from 



the next-to-last dynode pass through 
the anode grid. In more sophisticated 
photomultiplier designs this phe- 
nomenon may be suppressed by 
auxiliary grids that shield the anode 
from the effects of the impinging 
electron cloud. 

TRANSIT-TIME 
CONSIDERATIONS 

Photoeathode Transit-Time 
Difference 

A time parameter of interest is 
the photoeathode transit-time differ- 
ence, the time difference between the 
peak current outputs for simultane- 
ous small-spot illumination of differ- 
ent parts of the photoeathode. (In 
practice, of course, one small area 
at a time is illuminated and the 
spread in illumination-peak output 
intervals is measured.) In a planar- 
cathode design, the transit time is 
longer for edge illumination than for 
center illumination because of the 
longer edge trajectories and the 
weaker electric field near the edge 
of the photoeathode. The center-to- 
edge transit-time difference may be 
as much as 10 nanoseconds. The 
spherical-section photoeathode af- 
fords more uniform time response 
than the planar photocafhode be- 
cause all the electron paths are 
nearly equal in length; however, the 
transit time is slightly longer for 
edge trajectories than for axial tra- 
jectories because of the weaker elec- 
tric field at the edge. 

The photoeathode transit-time dif- 
ference is ultimately limited by 
the initial-velocity distribution of 
the photoelectrons; this distribution 
causes time-broadening of the elec- 
tron packet during its flight from 
the photoeathode to the first dynode. 
The^ broadening effect can be mini- 
mized by increasing the strength of 



26 



RCA Photomuitiplier Manual 



the electric field at the surface of the 
photocathode. 

Electron-Multiplier 
Time Response 

Because the energy spread of sec- 
ondary electrons is even larger than 
that of photoelectrons, initial-velocity 
effects are the major limitation on 
the time response of the electron 
multiplier. Multiplier time response 
is usually improved by the use of 
high electric-field strengths at the dy- 
node surfaces and compensated de- 
sign geometries. In a compensated 
design, such as that shown in Fig. 
22, longer electron paths and weaker 
fields alternate with shorter electron 
paths and stronger fields from dy- 
node to dynode to produce nearly 
equal total transit time. 

Excellent time response is achieved 
with crossed-field multipliers which 
use both magnetic and electro- 
static focusing fields. Crossed-field 




Fig. 22 — A compensated-design 
multiplier. 



multipliers yield nearly isochronous 
electron trajectories because of their 
compensation characteristics; the 
higher-velocity electrons have longer 
path lengths between stages, and 
thus arrive at the same time as elec- 
trons which began with lower initial 
velocities. Because this type of multi- 
plier must be operated at a unique 
voltage for a given magnetic field 
strength, gain is not adjustable un- 
less electromagnets are used. 

Certain materials have advantages 
over other materials in providing 
good multiplier time performance. 
For example, a standard dynode ma- 
terial, copper beryllium (CuBe), has 
a maximum gain per stage of 8 at 
a 600-volt interstage potential dif- 
ference. In contrast, gallium phos- 
phide (GaP) exhibits a gain of 50 
or more at a 1000-volt interstage 
potential difference. 10 The advantage 
in time performance of the GaP 
dynode over one of the CuBe type 
is that the number of stages, and thus 
the total transit time, may be re- 
duced, and that the energy spread of 
the secondary electrons is less. 

RUGGEDIZED 
PHOTOMULTIPLIERS 

New and increased demands on 
the capabilities of photomultipliers 
to survive severe vibrational environ- 
ments have resulted in the develop- 
ment of a series of ruggedized photo- 
multiplier types. The ruggedization. 
of these photomultipliers has been 
achieved without degradation of elec- 
trical characteristics. 

Ruggedization of glass-envelope 
photomultipliers has been accom- 
plished by moving the dynode cage 
extremely close to the stem (thereby 
drastically shortening the lead lengths 
and raising their mechanical resonant 
frequency), and by using heavier 
leads, extra spacers to hold the dy- 



Principles of Photomultiplier Design 



27 



node cage in place, and a special 
heavy-duty welding process on the 
metal-to-metal joints. A new type of 
tube, the stacked-ceramic photomul- 
tiplier, has been developed for use 
in the most severe mechanical en- 
vironments. In this type, each dynode 
is mounted on the inside of a flat 
metal ring which is separated from 
the next dynode ring by a ceramic- 
ring spacer. The metal rings and 
ceramic spacers are then brazed to- 
gether to form the vacuum envelope. 

Ruggedized tubes are tested for 
their ability to withstand sinusoidal 
and random vibrations and for their 
resistance to shock. Test-parameter 
values for RCA ruggedized photo- 
multipliers are given in the Data 
Section. 

The "sinusoidal vibration test" is 
performed on apparatus that applies 
a variable sinusoidal vibration to the 
tube. The sinusoidal frequency is 
varied logarithmically with time 
from a minimum to a maximum to a 
minimum value. Each tube is vi- 
brated in each of the three ortho- 
gonal axes shown in Fig. 23. The 
total time for the vibration of all 
three axes for ruggedized photomul- 
tipliers is given in the Data Section. 

The peak acceleration is expressed 
in units of the acceleration of gravity 
at the earth's axis (32.17 ft/s 2 ) and 
is denoted by the symbol g. The 
distance over which the tube is vi- 
brated is referred to as the "double 
amplitude." Double amplitude is the 
distance between the limits of travel 
of the tube in both directions of the 
sinusoidal vibration. 

The shock test is performed on 
apparatus that applies a half-wave 
sinusoidal shock pulse to the photo- 
multiplier tube. The tube is sub- 
jected to the shock in each of three 
orthogonal axes shown in Fig. 23. 

The shock pulse is expressed in 
terms of the peak acceleration of the 



pulse and the time duration of the 
pulse. The tubes may receive more 
than one shock pulse in each of the 
orthogonal axes. 




V 












/ 


Y _C 


\ 


^1 v , 


1 L 


: _) — _ 




* - 






■ ♦ 








A' 




























1 






I 



































x+ 

SECTION AA 1 



Z- 

Fig. 23 — Test axes used in random- 
vibration and shock testing of stacked- 
ceramic photomultipliers. 

BASING CONSIDERATIONS 

Photomultiplier tubes may have 
either a temporary or a permanently 
attached base. Dimensional outline 
diagrams such as those shown in 
Fig. 24 are provided in the Techni- 
cal Data Section of this manual and 
in individual tube bulletins. Indicated 
on the diagrams are "the type of base 
employed, maximum mechanical di- 
mensions, radii of curvature where 
applicable, pin/ lead details, location 
and dimensions of magnetic parts 
used (in tubes utilizing minimum 
number of magnetic materials), and 
notes regarding restricted mounting 
areas, again where applicable. 

Photomultiplier tubes intended to 
be soldered directly to circuit boards 
or housings are supplied with semi- 
flexible or "flying" leads and a tern- 



28 



RCA Photomultiplier Manual 



I4SEMIFLEXIBLE DUMET 
LEADS ,0I6±.004 DIA 
1.5 MIN. LENGTH 



TEMPORARY BASE 
JEDECNO.B20-I02- 




PHOTOCATHODE 



PROTECTIVE 

PLASTIC 

SHIELD 



FACEPLATE^ 


ffl 








PHOTOCATHODE"^ 

T6BULB^» 
BASE 


/.685 

i.035 

R. 

f 


! 


i 

A 


JEDECNo.EIZ-72 
AND 
PROTECTIVE -- 


P 


t 
R 

T 


SHELL H 


1 

— 


) t 


F 


i 


,J 









Dimensions 



Inches 

3.94 max. 



mm 



3.50 



C 
D 

E 
F 
H 
P 
R 



+.06 
-.12 
.5 min. dia. 
.78 max. dia. 
.755 max. dia. 
.38 max. 
.75 min. 
.30 max. 
1.0 max. 



100.0 max. 
88.9 j^' 5 

12.7 min. dia 

19.8 max. dia. 
19.18 max. dia. 
9.7 max. 

19.0 min. 
7.6 max. 
25 max. 



The dimensions in millimeters are derived from the basic inch dimensions 

(1 inch = 25.4 mm) 

Fig. 24 — Typical dimensional-outline drawings showing the type of base supplied 
with each tube and pertinent notes. 



porary base, intended for testing 
purposes only, that should be re- 
moved prior to permanent installa- 
tion. 

A lead-terminal diagram that 
shows photomultiplier-tube lead 
orientation with the temporary base 
removed, shown in Fig. 25, provides 

12 SEMIFLEXIBLE 
LEADS 
.016 + .004 DIA. 



a lead indexing reference. A lead- 
connection diagram, such as the one 
shown in Fig. 26, relates terminal to 




25.7' 



(NOTE 2) 




PIN I - DYNODE No. I 
PIN 2 -DYNODE No 3 
PIN 3-DYN0DENO.5 
PIN 4-DYNODEN0.7 
PIN 5 -DYNODE No.9 
PIN 6 - ANODE 
PIN 7 -DYNODE No. 10 
PIN B- DYNODE No. 8 
PIN 9 -DYNODE No. 6 
PINIO-OYNODENo.4 
PIN 1 1 -DYNODE No.2 
PIN 12 -PHOTOCATHODE 



LEAD I -DYNODE No. I 
LEAD 2- DYNODE N0.3 
LEAD 3-DYNODEN0.5 
LEAD 4- DYNODE No. 7 
LEAD 5 -DYNODE No.9 
) D '4 LEAD 6 - ANODE 

LEAD 7 - DYNODE No. 10 
LEAD 6- DYNODE No.8 
LEAD 9- DYNODE No.6 
LEADI0-DYNODENO.4 
LEAD 1 1 -DYNODE No.2 
LEAD 12- PHOTOCATHODE 



'DY 2 



Fig. 25 — Lead-orientation diagram. 



Fig. 26 — Lead-connection diagram: (a 

with base connected, (b) with tempora, 

base removed. 



Principles of Photomultiplier Design 



29 



electrode. Care must be exercised in 
interpreting basing and lead-terminal 
diagrams to insure against possible 
damage to the photomultiplier re- 
sulting from incorrect connections. 

REFERENCES 

1. H. E. Kulsrud, "A Programming 
System for Electron Optical 
Simulation," RCA Rev., Vol. 
28, No. 2, p. 351 (1967) 

2. R. W. Engstrom and R. M. 
Matheson, "Multiplier-Phototube 
Development Program at RCA- 
Lancaster," IRE Trans. Nucl. 
Sci., Vol. NS-7, No. 2-3, p. 52 
(1960) 

3. E. A. Taft and H. R. Philipp, 
"Structure in the Energy Distri- 
bution of Photoelectrons from 
K 3 Sb and Cs 3 Sb," Phys. Rev., 
Vol. 115, No. 6, p. 1583 (1959) 

4. D. E. Persyk, "Computer Simu- 
lation of Photomultiplier-Tube 
Operation," RCA Reprint PE- 
473 (1970) 



5. K. C. Schmidt and C. F. Hendee, 
"Continuous-Channel Electron 
Multiplier Operated in the Pulse- 
Saturated Mode," IEEE Trans. 
Nucl. Sci., Vol. NS-13, No. 3, 
p. 100 (1966) 

6. A. D. White and D. L. Perry, 
"Notes on the Use of a Rubber 
Membrane Model for Plotting 
Electron Trajectories," Rev. Sci. 
Instr., Vol. 32, No. 6, p. 730 
(1961) 

7. I. A. D. Lewis and F. H. Wells, 
Milli-Microsecond Pulse Tech- 
niques, Pergamon Press (1959) 

8. "Time-Domain Reflectometry," 
Hewlett Packard Application 
Note 62 

9. "Time-Domain Reflectometry," 
Tektronix Publication 062- 
0703-00 

10. G. A. Morton, H. M. Smith, Jr., 
and H. R. Krall, "The Perform- 
ance of High-Gain First-Dynode 
Photomultipliers," IEEE Trans. 
Nucl. Sci., Vol. NS-16, No. 1, 
p. 92 (1969) 



30 



Basic Performance 
Characteristics 



THIS section discusses the opera- 
tional characteristics of photo- 
multipliers that are listed in the 
Technical Data section of this Man- 
ual and in the technical data bulle< 
tins that describe each type of 
photomultiplier. The index at the 
back of the Manual gives the loca- 
tion of particular characteristics of 
interest within the pages of this sec- 
tion. 

CLASSES OF DATA 

Ratings 

Photomultiplier ratings are estab- 
lished to help equipment designers 
utilize the performance and service 
capability of each device to best ad- 
vantage. These ratings are based on 
careful study and extensive testing 
and indicate limits within which the 
operating conditions must be main- 
tained to insure satisfactory perform- 
ance. The maximum ratings given 
are based on the Absolute Maximum 
System, a system initiated by the 
Joint Electron Device Engineering 
Council (JEDEC) and subscribed to 
by the National Electrical Manufac- 
turers Association (NEMA) and 
Electronic Industries Association 
(EIA). 



Equipment Design Parameters 

Characteristic Range Values for 
Equipment Design are included in 
this Manual and in individual data 
bulletins in addition to the maximum 
ratings described above. Minimum, 
maximum, and typical values of se- 
lected parameters for a given tube 
of a particular type are listed under 
this heading to assist an equipment 
designer in selecting the proper de- 
vice for his application. Knowledge 
of parameter values allows the de- 
signer to specify the proper latitude 
of voltage and gain controls, addi- 
tional amplification, etc., which will 
permit optimum utilization of the 
device he has chosen. Of the para- 
meters listed, some are measured 
and some calculated from the mea- 
sured quantities. Not all parameters 
are described for every device be- 
cause many parameters are im- 
portant only in specific infrequent 
applications. 

RATINGS AND DESIGN 
PARAMETERS 

Voltage 

The maximum voltage ratings for 
photomultipliers are usually given as 



Basic Performance Characteristics 



31 



the maximum voltage to be applied 
between the anode and cathode, be- 
tween the anode and the last dynode, 
between consecutive dynodes, and 
between the cathode and the first 
dynode. A maximum anode-to-cath- 
ode voltage is specified so that the 
tube does not become regenerative. 
At excessive values of voltage the 
output current will become very 
noisy and may become so large as 
to exceed the current rating and 
cause damage to the secondary- 
emitting surfaces of the dynodes 
which could result in eventual tube 
failure. Maximum-voltage ratings are 
also specified between the last dy- 
node and the anode. This rating is 
usually made somewhat lower in 
value than the ratings between con- 
secutive dynodes to reduce the pos- 
sibility of breakdown between the 
last dynode and the anode as a re- 
sult of the close proximity of the 
two. Maximum voltages between 
consecutive dynodes are specified to 
prevent the possibility of breakdown 
between these elements and also to 
prevent excessive ohmic leakage in 
the region of the stem leads. The 
inter-dynode maximum voltage rat- 
ing maintains operation below the 
point of maximum secondary emis- 
sion above which the gain actually 
decreases. The specification of a 
maximum cathode-to-first-dynode- 
voltage rating also reduces the pos- 
sibility of leakage, breakdown, and 
reduction in secondary emission. 

The sum of the individual maxi- 
mum voltage ratings may exceed the 
maximum anode-to-cathode voltage 
rating. This statement does not mean 
that the over-all voltage rating can 
be exceeded, but that, depending 
upon the application, special volt- 
age distributions may be made. A 
detailed discussion of voltage distri- 
bution is given in the section on 
Voltage-Divider Considerations. One 



possible application in which special 
or "tapered" voltage distribution may 
be desirable is in equipment in which 
large anode pulse currents may be 
encountered. To reduce the possi- 
bility of space-charge limiting, it is 
often desirable to increase the volt- 
age between the last few successive 
dynodes and between the last dynode 
and the anode. It may also be de- 
sirable to increase the normal cath- 
ode-to-first-dynode potential when 
optimum pulse-height resolution is 
desired. 

Cathode Current 

The magnitude of the maximum 
permissible cathode current depends 
upon the type of photocathode and 
the type of substrate or conductive 
layer upon which the photocathode 
is placed. In a tube having a semi- 
transparent bialkali photocathode up 
to approximately one inch in di- 
ameter, a peak cathode current of 
1 X 10~ 8 ampere at a tube tem- 
perature of 22°C or 1 X 10- 10 
ampere at — 100"C should not be 
exceeded. Tubes having larger photo- 
cathodes should not be subjected to 
cathode currents exceeding 1 X 10 -9 
ampere at a tube temperature of 
22°C or 1 X 10- 11 ampere at 
— 100°C. Because of the resistivity 
of the photocathode, the voltage drop 
caused by higher peak cathode cur- 
rents could produce radial electric 
fields across the photocathode, and 
result in poor photoelectron collec- 
tion at the first dynode. Photo- 
surface resistivity also increases with 
decreasing temperature and thus re- 
duces the maximum permissible cur- 
rent still further. 

Opaque cesium-antimony photo- 
cathodes can be operated at peak 
cathode currents as high as 100 
microamperes per square centi- 
meter. Semitransparent cesium-anti- 



32 



RCA Photomultiplier Manual 



mony photocathodes (CsSb) should 
be operated at currents less than 1 
microampere per square centimeter 
at room temperature, and propor- 
tionally less as the ambient tempera- 
ture is reduced. Semitransparent 
multialkali and Ag-O-Cs (S-l) photo- 
cathodes have lower resistivities 
than the CsSb types and can be oper- 
ated at currents as high as 100 micro- 
amperes per square centimeter at 
room temperature; again, derating is 
required as temperature is reduced. 



Anode Current 

All photomultipliers have a maxi- 
mum anode current rating. The pri- 
mary reason for such a rating is to 
limit the anode power dissipation 
to approximately one-half watt or 
less. Consequently, the magnitude of 
the maximum anode current is re- 
stricted to a few milliamperes when 
the tube is operated at 100 to 200 
volts between the last dynode and 
the anode. Space-charge effects must 
also be considered in computation 
of the magnitude of the maximum 
recommended anode current. As the 
anode current increases, operation 
becomes nonlinear because of the 
space-charge buildup between the 
last few dynodes and the last dynode 
and the anode. Even though a photo- 
multiplier acts essentially as a con- 
stant-current device, the signal volt- 
age developed across the load is in 
series with the last-dynode-to-anode 
voltage and consequently opposes it. 
Nonlinear operation may occur as 
the load resistance and load current 
become large. 

Another possible adverse effect of 
operating a given photomultiplier at 
an excessively high anode current is 
the increased fatigue that occurs as 
the average anode current increases. 



Temperature 

A maximum ambient temperature, 
and in some instances a minimum 
temperature, is specified for all 
photomultipliers. The specification of 
maximum ambient temperatures re- 
duces the possibility of heat dam- 
age to the tube. Because dark current 
(noise) increases with temperature, it 
is possible that a point of instability 
may be reached at high temperatures 
at which the tube will break down 
and the resulting large currents 
permanently damage the secondary- 
emitting surfaces of the dynodes. 
Even if the dark current does not 
increase to a level that causes re- 
generation, the tube might be dam- 
aged as a result of changes in the 
composition of the photocathode and 
dynode materials. Cesium, for ex- 
ample, is very volatile and may be 
driven from the photocathode and 
dynode surfaces to condense on the 
cooler portions of the tube and 
thereby reduce gain and cathode 
sensitivity or both. 

It is recommended that photomul- 
tipliers be operated at or below 
room temperature so that the effects 
of dark current are minimized. The 
variation of dark current, or noise, 
is most important because of its ef- 
fect on ultimate low-light-level sensi- 
tivity. Various cryostats and solid- 
state thermionic coolers have been 
designed that reduce dark current 
at low temperatures in low-light-level 
applications. An important consider- 
ation in the use of these devices is 
to prevent condensation of moisture 
on the photomultiplier window. A 
controlled low-humidity atmosphere 
or special equipment configuration 
may be necessary to prevent such 
condensation. 

When the temperature is varied, 
small changes in the spectral re- 
sponse characteristics of the photo- 



Basic Performance Characteristics 



33 




200 



300 



400 500 600 

WAVELENGTH — NANOMETERS 



700 



800 



Fig. 27 — Temperature coefficient of cesium-antimony cathodes as a function 
of wavelength at 20'C. Note the large positive effect near the threshold. 



cathode may be observed. Fig. 27 
shows a temperature coefficient of 
sensitivity of a CsSb photocathode as 
a function of wavelength. 1 The in- 
crease of red sensitivity with tem- 
perature is typical for photoemission. 
Alhough it is recommended that 
photomultipliers be operated at or 
below room temperature, many de- 
vices have minimum operating tem- 
peratures. The materials used in 
producing photocathodes are semi- 
conductors. Their conductivity de- 
creases with decreasing temperature 
until the photocathode becomes so 
resistive that a sizable voltage drop 
may occur across the cathode sur- 
face when cathode current flows. 
Such a voltage drop may result in 
loss of linearity of the output cur- 
rent as a function of light level. Fig. 
28 shows the resistance per square 
of common semitransparent photo- 
cathode materials; measurements 
were made in special tubes having 
conections to parallel conducting 
lines on the photocathode. Semi- 



transparent photocathodes of the 
bialkali and cesium antimony types 
have particularly high resistivity. 
Consequently, the operating tem- 
peratures of these photocathodes 



'° 9 8 
e 

4 



X 6 

° A 



Ul 

o 

Siol 




, LIQUID N2 
»■ I I— 



JML I 



-200 -160 -120 -80 -40 40 
TEMPERATURE— DEGREES C 

Fig. 28 — Resistance per square as a 
function of temperature for the cesium- 
antimony and the multialkali semi- 
transparent photocathodes. These data 
were obtained with special tubes hav- 
ing connections to parallel conducting 
lines on the photocathode. 



34 



RCA Photomultiplier Manual 



should be kept above — 100°C, de- 
pending somewhat upon the photo- 
cathode diameter, the light-spot 
diameter, and the cathode current. 
The larger the photocathode diameter 
and the smaller the light-spot di- 
ameter, the more severe the effect. 
Opaque photocathodes and photo- 
cathodes having conductive sub- 
strates can be operated at cryogenic 
temperatures. 

Another reason for avoiding the 
operation of photomultipliers at ex- 
tremely low temperatures is the pos- 
sible phase change that this type of 
operation may cause in some of the 
metal parts. These changes are par- 
ticularly probable when Kovar is 
used in metal-to-glass seals. Tubes 
utilizing Kovar in their construction 
should not be operated at tempera- 
tures below that of liquid nitrogen 
(-196°C). 

In some tubes, particularly those 
with multialkali photocathodes, it is 
sometimes observed that the noise 
actually increases as the temperature 
of the photocathode is reduced be- 
low about -40°C. The reason for 
this noise increase is not understood. 
However, most of the dark-current 
reduction has already been achieved 
at temperatures above — 40°C. 

In general, it is recommended that 
all wires and connections to the tube 
be encapsulated for refrigerated 
operation. Encapsulation minimizes 
breakdown of insulation, especially 
that caused by moisture condensa- 
tion. 

Exterior Magnetic and 
Electrostatic Fields 

All photomultipliers are to some 
extent sensitive to the presence of 
magnetic and electrostatic fields. 
These fields may deflect electrons 
from their normal path between 
stages and cause a loss of gain. 



Tubes designed for scintillation 
counting are generally very sensitive 
to magnetic fields because of the 
relatively long path from the cathode 
to the first dynode; consequently, 
such tubes ordinarily require electro- 
static and magnetic shielding. Mag- 
netic fields may easily reduce the 
anode current by as much as 50 per 
cent or more of the "no-field" value. 
The three curves in Fig. 29 show 
the effect on anode current of mag- 
netic fields parallel and perpendicular 
to the main tube axis and parallel to 
the dynodes. The curves are usually 
provided for one or more values of 
over-all applied voltage and indi- 
cate the relative anode current in 
per cent as a function of magnetic 
field intensity in oersteds. Fig. 30 
shows the variation of output current 
of several photomultiplier tubes as 
a function of magnetic-field intensity 
directly parallel to the major axis of 
the tube. The magnitude of the effect 
depends to a great extent upon the 
structure of the tube, the orientation 
of the field, and the operating volt- 
age. In general, the higher the operat- 
ing voltage, the less the effect of 
these fields. 

High-mu material in the form of 
foils or preformed shields is avail- 
able commercially for most photo- 
multipliers. When such a shield is 
used, it must be at cathode potential. 
The use of an external shield may 
present a safety hazard because in 
many applications the photomulti- 
plier is operated with the anode at 
ground potential and the cathode at 
a high negative potential. Adequate 
safeguards are therefore required to 
prevent personnel from coming in 
contact with the high potential of 
the shield. The application of high 
voltage, with respect to the cathode, 
to insulating or other materials sup- 
porting or shielding the photomulti- 
plier at the photocathode end of the 



Basic Performance Characteristics 



35 



SUPPLY VOLTAGE E IS ACROSS A VOLTAGE 
DIVIDER PROVIDING 1/6 OF E BETWEEN 
CATHODE AND DYNODE No I ; 1/12 OF E FOR 
EACH SUCCEEDING DYNODE-STAGE; AND 1/12 
OF E BETWEEN DYNODE No.lO AND ANODE. 

PHOTOCATHODE IS FULLY ILLUMINATED. 
TUBE IS ORIENTED IN MAGNETIC FIELD AS 
SHOWN BELOW : 




POSITIVE VALUE OF H IN DIRECTION SHOWN: 
(l)-«-,(2) J ,(3)»* 

DIRECTION (3) IS OUT OF PAPER 





-100 



80 



60 



20 



; o 

: I00 



80 
60 




E=600V 



(3) 



E = 1500 V 




E=600V 



-15 -10 -5 +5 +10 +15 
MAGNETIC FIELD INTENSITY (H) — OERSTEDS 

Fig. 29 — Curves showing the effect on 

anode current of magnetic fields parallel 

and perpendicular to the main tube 

axis and parallel to the dynodes. 

tube should not be permitted unless 
the materials are of the type that 
will limit the leakage current to the 
tube envelope to 1 X 10- 12 ampere 
or less. In addition to increasing dark 



current and noise output because of 
voltage gradients developed across 
the envelope wall, the application of 
high voltage may produce leakage 
current to the cathode through the 
tube envelope and insulating ma- 
terials; this current can permanently 
damage the tube. 

It is possible to modulate the out- 
put current of a photomultiplier 
with a magnetic field. The application 
of a magnetic field generally causes 
no permanent damage to a photo- 
multiplier although it may magne- 
tize those internal parts of a tube 
that contain ferromagnetic materials 
(tubes are available which contain 
practically no ferromagnetic ma- 
terials). If tube parts do become mag- 
netized, the performance of the tube 
may be degraded somewhat; how- 
ever, the condition is easily corrected 
by degaussing, a process in which a 
tube is placed in and then gradually 
withdrawn from the center of a coil 
operated at an alternating current 
of 60 Hz with a maximum fiield 
strength of 100 oersteds. 

Environments of Excessive 
Vibration or Shock 

Most photomultipliers will survive 
only a reasonable amount of shock 
or vibration (less than 10 -g shock, 
depending on shock duration and di- 
rection). Although special photomul- 
tipliers have been designed to sur- 
vive in extreme environments (shock 
values from 30 to 1500 g), the user 
should make every effort to avoid 
excessive shock or vibration, possi- 
bly by the use of special vibration- 
isolation fixtures. The photomultiplier 
tube should be handled as the deli- 
cate instrument that it is. Excessive 
shock or vibration can actually cause 
physical damage to the tube to the 
point of shorting out some of the 
elements or even resulting in break- 



36 



RCA Photomultiplier Manual 



H PARALLEL TO MAJOR AXIS OF TUBE (POSITIVE VALUES FOR LINES 
OF FORCE TOWARD TUBE BASE) 




-30 



-20 -10 0_ 

MAGNETIC FIELD INTENSITY (H)- 



10 

OERSTED 



fig. 30 — Variation of output current of several photomultiplier tubes as a func- 
tion of magnetic-field intensity directly parallel to the major axis of the tube. 



age of the envelope and loss of 
vacuum. A lesser degree of shock 
may result in deformation of the ele- 
ments of the tube which can result 
in loss of gain or deterioration of 
other performance parameters. If 
measurements are being made while 
a tube is vibrated, it is likely that 
the output will be modulated by the 
vibration not only because the light 
spot may be deflected to different 
positions on the photocathode but 
because some of the dynodes may 
actually vibrate and cause modula- 
tion of the secondary-emission gain. 
Many photomultipliers have been 
designed for use under severe en- 
vironmental conditions of shock and 
vibration and, in many cases, spe- 
cifically for use in missile and rocket 
applications. Such tubes, however, 
find uses in many other applications 
including oil-well logging or other 
industrial control appulications where 
the tube may be subjected to rough 
usage. These tubes are available 
with most of the electrical and spec- 
tral characteristics typical of the 
more-general-purpose types. These 
types differ primarily in mechanical 
construction in that additional sup- 
porting members may be employed 



and an improved cathode connection 
may be used to insure positive con- 
tact when the tube is subjected to 
these environments. 

Tubes recommended for use un- 
der severe environmental conditions 
are usually designed to withstand en- 
vironmental tests equivalent to those 
specified in the applicable portions 
of MIL-E-5272C or MIL-STD-810B 
in which the specified accelerations 
are applied directly to the tubes. 
Certain other types of photomulti- 
pliers, particularly those utilizing 
stacked ceramic-to-metal brazed con- 
struction, have an environmental 
capability in excess of those specified 
in the above military specifications. 
These types find use in applications 
in which the tube must not only sur- 
vive but operate reliably during 
periods of extreme shock and vibra- 
tion such as those encountered dur- 
ing an actual rocket launch. 

Unusual Atmospheric 
Environments 

Photomultipliers may be operated 
in environments in which the pres- 
sure varies from that of a nearly per- 
fect vacuum such as would be 



Basic Performance Characteristics 



37 



encountered in space to several at- 
mospheres as perhaps would be pres- 
ent in an underwater application. 
Although photomultipliers them- 
selves may operate at very low pres- 
sures, precautions must be taken to 
reduce the possibility of breakdown 
between external tube and circuit 
elements in partial-pressure environ- 
ments as predicted by Paschen's Law. 
The gas contributing to the partial 
pressure could be the result of out- 
gassing of encapsulating and other 
materials used external to the photo- 
multiplier. 

High-pressure operation of photo- 
multipliers depends primarily upon 
the physical size and strength of the 
materials employed in their construc- 
tion. Although a two-inch-diameter 
tube employing stacked ceramic-to- 
metal brazed construction can easily 
withstand 4 or 5 atmospheres, other 
all-glass two-inch-diameter tubes 
with very thin entrance windows are 
limited to only slight additional load- 
ing above atmospheric pressure. 

Helium Penetration. Because he- 
lium permeates through glass, the 
operation or storage of photomulti- 
pliers in environments where helium 
is present must be avoided. Tubes 
subjected to such an environment 
will exhibit a noise increase and 
possibly an increase in afterpulsing. 
Depending upon the degree of 
permeation, a point will soon be 
reached at which complete ioniza- 
tion takes place and makes the tube 
unusable. Continued operation will 
result in the deterioration of the 
photocathode response and lead to 
an inoperative tube. 

PHOTOMULTIPLIER DATA 

Sensitivity 

Sensitivity provides an indication 
of the magnitude of the signal that 



may be expected for a given light 
input. Photocathode and anode sensi- 
tivities are stated in terms of lumi- 
nous or radiant sensitivity. 

Current amplification is stated in 
terms of the ratio of anode output 
current to photocathode current and 
is measured at specified values of 
electrode voltage. Typical photomul- 
tiplier current-amplification values 
range from 10 4 to 10 9 when the 
tubes are operated at the voltages 
recommended for the number of 
stages and dynode materials used. 

Fig. 31 describes the anode sensi- 
tivity in amperes per lumen and the 
typical amplification characteristics 
of a photomultiplier as a function of 
the applied voltage. These curves 



SUPPLY VOLTAGE (E) ACROSS VOLTAGE DIVIDER 


PROVIDING l/IOOFE BETWEEN CATHODE AND 


DYNODE No.l ; 1/10 OF E FOR EACH SUCCEEDING 


DYNODE; AND l/IO OF E BETWEEN DYNODE 


No.9 8 ANODE. 




8 


Li i i i i i i i 


i_ 


8 


6 




- 


6 


— 4 




_ 


4 


* 








S 2 




_ 


2 


00 








CM 








cu|0 3 
§ 8 


= / / 


- 


I0 7 

8 


1- 6 


: /v 




6 2 


9< 
o 


- 


4 2 

t- 
< 


5 * 


Jv / 




2 1 


UJ 

In* 


#> / 




I0 6 | 


a. 


- #^# / 




e < 

6 h- 

z 

4 UJ 

a: 

<r 

3 


< 2 

>- 
tio 

> 8 
1- 6 


- / */w # 

/ ' <Jv v/ 

7 /y £/ 




2 <-> 

I0« 

8 
6 


■Z. 4 


-// # 

j / y 




4 


to 

2 




2 


1 


/ 1 i / 1 i i i i i 


i 


10" 



500 600 80 ° 1000 l20 ° 1500 

SUPPLY VOLTS (E) BETWEEN ANODE AND CATHODE 

Fig. 31 — Typical anode sensitivity and 
amplification characteristics of a photo- 
multiplier tube as a function of 
applied voltage. 



38 



RCA Photomultiplier Manual 



illustrate the wide range of ampli- 
fication of which a photomultiplier 
is capable, provided a well regulated 
voltage power supply is available. 
It is customary to present these 
sensitivity, amplification, and voltage 
data on logarithmic scales that cover 
the normal range of operation; the 
resultant curves are then closely ap- 
proximated by straight lines. Curves 
of minimum and maximum sensitiv- 
ity are shown, as well as those of 
the typical sensitivity and amplifi- 
cation. 

Luminous sensitivity is defined as 
the output anode current divided by 
the luminous flux incident on the 
photocathode, measured at a spe- 
cified electrode voltage; it is ex- 
pressed in amperes per lumen, A/lm. 
Luminous sensitivity is most fre- 
quently stated in terms of a tungsten- 
filament light source in a lime-glass 
envelope operated at a color tem- 
perature of 2854°K. A typical lumi- 
nous flux for test purposes is in the 
range of 10~ 8 to 10- 3 lumen. 

Cathode luminous sensitivity is 
the photocurrent emitted per lumen 
of incident light flux on the photo- 
cathode at constant electrode volt- 
age, and is also expressed in terms 
of amperes per lumen, A/lm. In the 
measurement of cathode luminous 
sensitivity in a photomultiplier, a dc 
voltage of 100 to 250 volts is applied 
between the cathode and all other 
electrodes connected together as an 
anode. Light flux in the order of 
10 -2 to 10 -4 lumen is then applied, 
and the measured photocurrent is 
divided by the specified light level 
to yield the required amperes per 
lumen. 

It is customary in sensitivity 
measurements to subtract any dark 
current which may be present so 
that only the effect of the radiation 
is recorded. 



Radiant sensitivity is the output 
anode current divided by the inci- 
dent radiant power at a given wave- 
length, measured at a constant 
electrode voltage. Cathode radiant 
sensitivity is the amount of current 
leaving the photocathode divided by 
the incident radiant power at a given 
wavelength, usually the wavelength 
of peak response. Values at other 
wavelengths can be computed from 
the relative-spectral-response curve. 
Alhough radiant sensitivity may be 
measured directly with suitable 
monochromatic sources and radia- 
tion-measuring equipment, it is 
usually computed from the measured 
anode and cathode luminous-sensi- 
tivity values and expressed in terms 
of amperes per watt, A/W. This 
computation uses conversion factors 
and is dependent upon established 
typical spectral-response character- 
istics. 

Some users prefer to use integral 
radiant sensitivity instead of lumi- 
nous sensitivity because the latter 
expression, although very commonly 
used, does have an element of in- 
appropriateness especially when used 
to express the sensitivity of devices 
which have sensitivity beyond the 
visible range of radiation. 

Integral radiant sensitivity is the 
ratio of the anode or photocathode 
current to the total radiation in watts 
impinging on the photocathode from 
a tungsten test lamp operating at a 
color temperature of 2854°K. Be- 
cause it is very difficult to calibrate 
a test lamp in total radiant watts, it 
is customary to use a luminous cali- 
bration and a conversion factor for 
the lamp of 20 lumens per watt. 

Another sensitivity parameter 
often specified for tubes, especially 
those intended for scintillation de- 
tectors, is blue sensitivity. Because 
most scintillators produce radiation 
in the blue region of the spectrum, 



Basic Performance Characteristics 



39 



tubes intended for these applications 
are processed for maximum sensi- 
tivity in this region. It has been 
found, during processing of such 
photomultipliers, that the maximum 
sensitivity for blue and for white 
light often does not occur at the 
same time. The anode blue sensitivity 
is measured with rated voltages ap- 
plied to the electrodes and a source 
of blue light incident upon the cath- 
ode. The blue-light source has a 
spectral output as indicated in Fig. 
32 and consists of the flux of tung- 
sten radiation at a color temperature 



SPECTRAL CHARACTERISTIC OF LIGHT FROM 
2854»K SOURCE AFTER PASSING THROUGH 
BLUE FILTER (CORNING C.S. No.5-58 
POLISHED TO 1/2 STOCK THICKNESS). 

MAXIMUM FILTER TRANSMISSION OCCURS AT 
430 NANOMETERS AND IS 60 PER CENT. 




fluxes range between 10 -5 and 10~ 7 
lumen, depending upon the tube 
sensitivity, when the anode blue 
sensitivity is measured, and between 
10~ 2 and 10~ 4 lumen when the cath- 
ode blue sensitivity is measured. 
Blue sensitivity is expressed in am- 
peres per "blue" lumen. 

A spectral response curve pro- 
vides the most complete specifica- 
tion of the sensitivity of a photo- 
cathode. Fig. 33 shows a set of 
typical spectral-response character- 
istics. Spectral-response characteris- 
tics may be given in terms of 
absolute radiant sensitivity, relative 
sensitivity, or quantum efficiency as 
a function of wavelength. These 
quantities are discussed in detail in 
the following paragraphs. 

The uppermost curve of Fig. 33, 
the relative-sensitivity curve, shows 
the per-cent relative sensitivity of the 



100 
8 
6 



sis 

a. E Q_ 
I I I 

t- to 

2Z1L 
IJUJU- 
w WW 



wr: - 



6 



300 



500 



350 400 450 

WAVELENGTH — NANOMETERS 

Fig. 32 — Spectral output of a blue 
light source used in measuring anode 

blue sensitivity. 
of 2854°K which is transmitted 
through a blue filter. Sensitivities are 
termed "blue" but are expressed in 
terms of the light flux which is in- 
cident upon the filter. Incident light 







100 300 500 700 

WAVELENGTH— NANOMETERS 

Fig. 33 — A set of typical spectral- 
response characteristics. 



40 



RCA Photomultiplier Manual 



peak sensitivity at each wavelength 
over the useful spectral range of the 
device. The center curve provides 
the absolute radiant sensitivity in 
units of milliamperes per watt. The 
third curve shows the per-cent quan- 
tum efficiency at each wavelength. 

The wavelength of peak response 
and the long-wavelength cutoff are 
primarily functions of the photo- 
cathode material. The short-wave- 
length cutoff is primarily a function 
of the window material. Each win- 
dow material has its characteristic 
cutoff region that varies from about 
300 nanometers for commonly used 
lime glass to 105 nanometers for 
lithium fluoride. 

Cathode quantum efficiency may 
be defined as the average number of 
photoelectrons emitted per incident 
photon. The cathode quantum effi- 
ciency, QE, in per cent at any given 
wavelength can be calculated from 
the following formula: 



QE 



1239 5 
s izjy£ (loQ) 



where S is the cathode radiant sensi- 
tivity at the wavelength \ in amperes 
per watt, and \ is the wavelength in 
nanometers. From this relation and 
the absolute (radiant) spectral-re- 
sponse characteristic it is possible to 
calculate the typical quantum effi- 
ciency at any wavelength within the 
spectral range of the photomultiplier 
under consideration. 

The relative spectral response can 
also be used to calculate cathode 
quantum efficiency if the radiant 
sensitivity at the wavelength of peak 
response is known. This calculation 
is made by multiplying the peak 
radiant sensitivity by the percentage 
of the peak response (obtained from 
the relative-response curves) for the 
wavelength in question. The resultant 
product is the absolute (radiant) 



spectral sensitivity for the desired 
wavelength. 

Dark Current 

Current flows in the anode circuit 
of a photomultiplier even when the 
tube is operated in complete dark- 
ness. The dc component of this cur- 
rent is called the anode dark current, 
or simply the dark current. This cur- 
rent and its resulting noise com- 
ponent usually limit the lower level 
of photomultiplier light detection; 
as a result, the anode dark-current 
value is nearly always given as part 
of the data for any tube. 

It is important to understand the 
variation of dark current in the 
photomultiplier as a function of vari- 
ous parameters so that the ultimate 
in low-light-level detection can be 
achieved. Although not all the 
mechanisms by which dark current 
is generated within a photomultiplier 
are understood, they can be cate- 
gorized by origin into three types: 
ohmic leakage, emission of electrons 
from the cathode and other elements 
of the tube, and regenerative effects. 

Interior ohmic leakage, the pre- 
dominant source of dark current at 
low operating voltage that can be 
identified by its proportionality with 
applied voltage, results from the im- 
perfect insulating properties of the 
glass stem, the supporting members, 
or the base, and is always present 
but usually negligible. Contamina- 
tion consisting of dirt, water vapor, 
or grease on the outside of the tube 
may also contribute to ohmic leak- 
age. A source of leakage inside the 
tube may be residual alkali metals 
used in the photomultiplier process- 
ing. 

As the operating voltage of the 
photomultiplier increases to mid- 
range values, the dark current begins 
to follow the gain characteristic of 



Basic Performance Characteristics 



41 



the tube. The source of this gain- 
proportional dark current is the dark 
or thermionic emission of electrons 
from the photocathode or the dy- 
nodes. Because each thermionically 
generated electron emitted from the 
photocathode is multiplied by the 
secondary-emission gain of the tube, 
the output is a pulse having a mag- 
nitude equal to the charge of one 
electron multiplied by the gain of 
the tube. This process, involving sta- 
tistical variations, is discussed in de- 
tail in the section on Statistical 
Fluctuation and Noise. Because the 
emission of thermionic electrons is 
random in time, the output dark cur- 
rent consists of random unidirec- 
tional pulses. The time average of 
these pulses may be measured on 
a dc meter and is usually the main 
component of the dc dark current at 
normal operating voltages. The vari- 
able nature of the thermionic dark 
current limits the sensitivity of a 
tube at very low light levels. It is 
not possible to balance out the ef- 
fect of this current in the same man- 
ner as the effect of a steady dc 
current resulting from ohmic leak- 
age. It is usually advantageous to 
operate a photomultiplier in the 
range where the thermionic com- 
ponent is dominant because in this 
range the signal-to-noise ratio of the 
tube is maximum. 

Regenerative effects occur in all 
photomultipliers at higher dynode 
voltages and cause the dark current 
to increase and become very er- 
ratic. At times the dark current may 
increase to the practical limitations 
of the tube and circuit. Permanent 
damage may be caused to the sensi- 
tized surfaces. 

Regenerative effects may be trig- 
gered by the electrostatic potential 
of the walls surrounding the photo- 
cathode and dynode-structure re- 
gion. As shown in Fig. 34, a positive 



H00O 




-800 -600 -400 -200 
EXTERNAL SHIELD VOLTS 
RELATIVE TO ANODE VOLTS 



Fig. 34 — Effect of external-shield po- 
tential on the noise of a 1P21 photo- 
multiplier. Note the desirability of 
maintaining a negative bulb potential. 

potential with respect to the cathode 
on the envelope wall can cause noisy 
operation by contributing to the gen- 
eration of minute ionic currents 
which flow through the glass and 
produce light at the cathode as a 
result of fluorescence within the 
glass. This noise need not arise from 
direct connection of the envelope 
wall to a positive potential; close 
proximity of the tube to such a po- 
tential will yield the same effect. In 
addition to the increased dark cur- 
rent and noise, the life of the photo- 
cathode is usually shortened when 
regenerative effects are present. To 
prevent this deterioration, the en- 
velope wall should be maintained 
near photocathode potential by 
wrapping or painting it with a con- 
ductive material and connecting this 
material to cathode potential. The 
connection is usually made through 
a high impedance to reduce the shock 
hazard. Insulating materials must be 
selected to limit the leakage currents 
in the vicinity of the photocathode 
to less than 1 X 10 -12 ampere. 

Excess noise or dark current can 
also result from a number ot other 
sources: fluorescent effects produced 
on the inner surfaces of the bulb 
during high-current operation, light 
generated within the tube at the 



42 



RCA Photomultiplier Manual 



various dynodes and at the anode as 
a result of electrons striking these 
elements, field emission occurring at 
sharp points within the tube, ionic 
bombardment of the photocathode 
as the result of minute quantities of 
residual gases, and scintillations in 
the glass envelope of the tube caused 
by radioactive elements within glass 
(most glasses contain some radio- 
active K 40 ). 

Photomultipliers with bialkali 
photocathodes, and to a lesser ex- 
tent those with cesium antimony 
photocathodes, exhibit a temporary 
increase in dark current and noise 
by as much as three orders of mag- 
nitude when the tube is exposed mo- 
mentarily to blue light or ultraviolet 
radiation from sources such as 
fluorescent room lighting, as shown 
in Fig. 35. This increase in dark 
current occurs even though voltage 
is not applied to the tube, and may 
persist for a period of from 6 to 24 
hours after such irradiation. 




0.1 I 10 lO* 10^ Kf 

TIME— MINUTES 

Fig. 35 — Variation of dark current 
following exposure of photocathode to 
cool white fluorescent-lamp radiation. 
The various photocathodes are identified 
by their spectral-response symbols. 



Dark-Current Specification. Dark- 
current values are often specified at 
a particular value of anode sensitiv- 
ity rather than at a fixed operating 
voltage. Specifications of dark cur- 
rent in this manner are more closely 



related to the actual application of 
the photomultiplier. 

The best operating range for a 
given photomultiplier can usually be 
predicted from the quotient of the 
anode dark current and the luminous 
sensitivity at which the dark current 
is measured. This quotient is iden- 
tified as the Equivalent Anode Dark 
Current Input (EADCI) in the 
Technical Data section of this Man- 
ual and in individual tube bulletins, 
and is the value of radiant flux inci- 
dent on the photocathode required 
to produce an anode current equal 
to the dark current observed. The 
units used in specifying EADCI are 
either lumens or watts. 

The curves in Fig. 36 show both 
typical anode dark current and 
equivalent anode dark current input 
(EADCI) as functions of luminous 



LUMINOUS SENSITIVITY IS VARIED BY ADJUST- 
ING THE SUPPLY VOLTAGE (E) ACROSS VOLT- 
AGE DIVIDER WHICH PROVIDES 1/10 OF E 
PER STAGE. 

LIGHT SOURCE IS A TUNGSTEN-FILAMENT 
LAMP OPERATED AT A COLOR TEMP 0F2854°K 

TUBE TEMPERATURE*22°C 




LUMINOUS SENSITIVITY-AMPERES/LUMEN 

1 1 1 1— I— 

1100 1300 1500 1700 1800 

SUPPLY VOLTAGE(E) — VOLTS 

Fig. 36 — Typical and equivalent 

anode-dark-current input (EADCI) as 

a function of luminous sensitivity. 



Basic Performance Characteristics 



43 



sensitivity. These curves can be used 
to predict the best operating range 
for a tube by considering the ratio 
of the dark current to the output 
sensitivity. The optimum operating 
range occurs in the region of the 
minimum on the EADCI curve, the 
region in which the signal-to-noise 
ratio is also near its maximum. The 
increase in the EADCI curve at 
higher values of sensitivity indicates 
the onset of a region of unstable and 
erratic operation. Many curves of 
this type also include a scale of 
anode-to-cathode supply voltage cor- 
responding to the sensitivity scale. 

Dark-Current Reduction. Dark 
current may be reduced by cooling 
the photomultiplier in a refrigerant, 
such as dry ice. This procedure is 
recommended in those applications 
in which maximum current amplifi- 
cation with minimum dark current is 
required. It should be noted, how- 
ever, that at reduced temperature 
the resistance of the cathode in- 
creases and may reduce the maxi- 
mum peak anode current. 

If care is taken to avoid damage 
to the photomultiplier by operation 
with excessive current, the dark cur- 
rent can often be reduced by a 
process of operating the photomulti- 
plier in the dark at or near the maxi- 
mum operating voltage. This process, 
called dark aging, may require sev- 
eral hours to several days. After such 
a process of aging, it is recommended 
that a photomultiplier be operated 
for several minutes at the reduced 
voltage before measurements are at- 
tempted. 

Equivalent Noise Input. Dark cur- 
rent is the average or dc value of 
discrete pulses occurring at random 
intervals. The fluctuations or noise 
associated with these pulses limit the 
measurement precision. Noise is 
evaluated in terms of a signal-to- 
noise-ratio measurement under spe- 



cified operating conditions. If the 
type of modulation and bandwidth 
used in the measurement is known, 
an equivalent noise input, ENI, can 
be calculated from the signal-to- 
noise ratio. Equivalent noise input 
is defined as the value of incident 
luminous or radiant flux which, 
when modulated in a stated man- 
ner, produces an rms output cur- 
rent equal to the rms noise current 
within a specified bandwidth, usually 
1 Hz. 

To determine the ENI for any 
other bandwidth, it is necessary only 
to multiply the published ENI value 
by the square root of that band- 
width. The conditions under which 
a value of ENI is obtained are simi- 
lar to those used to measure lumi- 
nous sensitivity, except that the light 
incident upon the photocathode is 
modulated by a light chopper so that 
a square-wave signal with equal on 
and off periods is produced at the 
photomultiplier output. 

It is of interest to consider what 
ENI may be expected for a photo- 
multiplier if it is assumed that the 
only source of noise is the photo- 



cathode dark emission, 



If the 



photocathode sensitivity is given by 
S in amperes per lumen aqd the un- 
modulated light flux by F in lumens, 
the unmodulated output signal is 
equal to SF^i, where (i is the gain 
of the multiplier. When the light is 
ohopped in an equal on and off 
square wave with its peak amplitude 
the same as that of the unmodulated 
light flux, the rms signal output at 
the fundamental frequency (with a 
narrow-bandpass measuring device) 
is SF /(.(2) 1 / 2 lit. The rms noise cur- 
rent from the dark emission in a 
band pass of Af is given by the ex- 
pression /i(2ei,Af) 1/2 , where e is the 
charge of an electron. This expression 
neglects the noise increase from dy- 
node amplification statistics which 



44 



RCA Photomultiplier Manual 



may increase this noise current by 
a few per cent. (See the section on 
Statistical Fluctuation and Noise.) 
In an evaluation of the expression 
for ENI, the signal must be equated 
to the dark noise, as follows: 

— = /MZeitAf) 1 / 2 

IT 

The value of ENI is then approxi- 
mately the value of F for the condi- 
tion of this equation: 



ENI = F =• 



iKeitAf) 1 ' 2 



For Af = 1, i t = 10- 1S ampere (a 
reasonably low value of photocathode 
dark-emission current), and S = 100 
microamperes per lumen, the ENI 
is equal to 4 X 10 -13 lumen. This 
value is equivalent to a photocathode 
peak signal current of 4 X 10 -17 
ampere. Because this signal current 
is almost two orders of magnitude 
less than the photocathode dark- 



emission current, the noise in the 
signal current is negligible. 

Because the dark emission is re- 
duced as the temperature of the 
photocathode is reduced, ENI may 
be reduced by cooling the photomul- 
tiplier. Fig. 37 shows the variation 
of ENI for a 1P21 (opaque CsSb 
photocathode) over a wide range of 
temperature. 

The differential dark-noise spec- 
trum shown in Fig. 38 displays the 
number of pulses counted per unit 
of time as a function of their height. 
Such a spectrum indicates the num- 
bers of single and multiple electron 
events originating at the photo- 
cathode. Submultiple electron emis- 
sion results from electron emission 
at one of the dynode surfaces. 

A differential dark-noise spectrum 
is obtained with a multichannel 
pulse-height analyzer. The calibra- 
tion of the single-photoelectron 
pulse height is determined by il- 
luminating the photocathode with a 
light level so low that there is a 



to 10 



5 io-'! 

a. 6 

5 4 

111 

CO 

m 6 

-I 4 

§ 



2 — 



IO" 1 



100 VOLTS PER STAGE. 

BANDWIDTH: I Hz 

LIGHT SOURCE: TUNGSTEN , AT 2854°K INTERRUPTED AT 90 Hz TO PRODUCE 
PULSES ALTERNATING BETWEEN ZERO AND FLUX VALUE SHOWN FOR ANY GIVEN 
TUBE TEMPERATURE; "ON" PERIOD OF PULSE EQUAL TO "OFF" PERIOD; RMS 
SIGNAL CURRENT = RMS NOISE CURRENT 

EXTERNAL SHIELD VOLTS RELATIVE TO ANODE VOLTS = -1000 




-100 -50 

TUBE TEMPERATURE — DEGREES CENTIGRADE 



+ 50 



Fig. 37 — Equivalent noise input in lumens for a 1P21 photomultiplier as a function 

of temperature. 



Basic Performance Characteristics 



45 



very low probability of coincident 
photoelectron emission. The dark- 
pulse distribution is then subtracted 
from the subsequent combination of 
dark pulses and single-photoelectron 
pulses, so that the remainder repre- 
sents only that distribution resulting 
from single-photoelectron events. By 
adjusting the gain of the pulse-height 
analyzer, the single-electron photo- 
peak can be placed in the desired 
channel to provide a normalized 
distribution. 

The dark-pulse spectrum of Fig. 
38 is characteristic of photomulti- 
pliers intended for use in scintilla- 
tion counting and other low-light- 
level pulse applications. The dashed 
portion at the left of the curve is an 



SUPPLY VOLTAGE (E) = 2500 VOLTS. 

DASHED PORTION INDICATES LOCATION OF 
SINGLE PHOTOELECTRON PEAK AND IS 
NORMALIZED TO COINCIDE WITH THE DARK 
PHOTOELECTRON PEAK. 

THIS CURVE WAS MEASUREP WITH A LOW 
INTENSITY LIGHT SOURCE TO INSURE LOW 
PROBABILITY OF COINCIDENT PHOTOELEC- 
TRON EMISSION. DARK PULSES WERE SUB- 
TRACTED. 

SOLID-LINE PORTION INDICATES DARK-PULSE 
SPECTRUM. 

TUBE TEMPERATURE * 22 °C 

ONE -PHOTOELECTRON PULSE HEIGHT = 4 
COUNTING CHANNELS. 

INTEGRATING TIME CONSTANT « IO/.S 
(R=IOKfl C'lOOOpF) 



i — r 

32 

7£j = 6.0 x 10* COUNTS/MIN. 

I PHOTOELECTRON 



^= 2 .2 x | o 4 COUNTS/MIN. 
4 PHOTOELECTRON 




I I I i I I, , I, I „ i I „ft 



2 4 6 

DARK-PULSE HEIGHT - 



8 10 12 

PHOTOELECTRONS 



Fig. 38 — Typical dark-noise pulse 
spectrum. 



extrapolation to show the location 
of the single photoelectron peak; the 
solid portion shows the actual dif- 
ferential dark-noise spectrum. The 
rapid change in slope of this curve 
in the pulse-height region of less 
than one-half photoelectron is as- 
sumed to be the result of electron 
emission from the first- and second- 
dynode surfaces. 

The slope of the curve for the 
pulse-height region between 1 and 
4 photoelectrons is as expected for 
single-electron emission, when the 
statistical nature of secondary-emis- 
sion multiplication is considered. The 
number of pulses in this region may 
be reduced by cooling the photo- 
multiplier. 

The slope of the curve for the 
pulse-height region greater than 4 
photoelectrons is presumed to be 
caused by multiple-electron-emission 
events. These multiple pulses are 
caused by processes such as ionic 
bombardment of the photocathode. 
Other mechanisms contributing to 
the noise spectrum include cosmic 
rays, field emission, and radioactive 
contaminants that produce scintilla- 
tions within the glass envelope. Cool- 
ing has little effect upon reduction 
of the number of these multiple- 
electron pulses, but extended opera- 
tion of the tube _may improve 
performance. Operation of the tube 
may result in erosion of sharp points 
and reduce the possible contribution 
of field effects. In addition, improve- 
ment occurs because residual gases 
are absorbed within the tube, ion 
bombardment of the photocathode is 
reduced, and the resulting multiple 
secondary-electron emission is les- 
sened. 

For many applications it is useful 
to have a summation of the total 
number of dark pulses. In Fig. 38, 
for example, the sum of dark-pulse 
counts from one electron equivalent 



46 



RCA Photomuttiplier Manual 



height to 32 electron equivalent 
heights is 3 x 10 4 counts per min- 
ute. The value of 32 is chosen to 
be essentially equivalent to infinity. 
Another useful figure is the summa- 
tion of pulses in the range from 4 
to 32 equivalent electron heights. 
This sum for this particular distri- 
bution is 2.2 X 10 3 counts per min- 
ute and corresponds to the rate of 
multiple-electron events. 

Pulse Counting 

Pulse-Height Resolution. An im- 
portant application of photomulti- 
pliers is in scintillation counting. 
(For a more detailed account of 
scintillation counting, see the section 
on Photomultiplier Applications.) In 
many scintillation-counting applica- 
tions, a photomultiplier is coupled to 
a thallium-activated sodium iodide 
NaI(Tl) crystal in which scintilla- 
tions are produced by gamma rays 
resulting from nuclear disintegra- 
tions. Because the output of a photo- 
multiplier is linear with light input 
and because the light energy of scin- 
tillations is directly proportional to 
the gamma-ray energy over a certain 
range, an electrical pulse is obtained 
which is a direct measure of the 
gamma-ray energy. Consequently, an 
important requirement of photomul- 
tipliers used in nuclear spectrometry 
is the ability to discriminate between 
pulses of various heights. The para- 
meter indicating the ability of a 
tube to perform this discrimination 
is called pulse-height resolution. 

A typical pulse-height distribution 
curve obtained with a Cs 137 source 
and an NaI(Tl) crystal is shown in 
Fig. 39. The main peak of the curves 
at the right is associated with mono- 
energetic gamma rays which lose 
their entire energy by photoelectric 
conversion in the crystal. Pulse- 
height resolution is defined as the 





PULSE HEIGHT RESOLUTION IN PER CENT 




IS DEFINED AS 100 TIMES THE RATIO OF 




THE WIDTH OF THE PHOTOPEAK AT HALF 




THE MAXIMUM COUNT RATE IN THE 




PHOTOPEAK HEIGHT (A) TO THE PULSE 




HEIGHT AT MAXIMUM PHOTOPEAK 


-J 


COUNT RATE (8). 


§ 

or 

UJ 

H 


1 PHOTO 


<0? 


(1 PEAK 


z h 






§1 




T — A 


OUJ 






I 3: 






liiij 














\ / 




ZK 






3UI 






oo- 






<->iu 






li-t- 






ou 






u>2 






3= 






UJ 


- rULot. ntlum a n 




Q. 





Fig. 39 — Typical pulse-height distribu- 
tion curve. 

width of the photopeak at half the 
maximum count rate divided by the 
pulse height at maximum count rate. 
Consequently, the lower the pulse- 
height resolution, the greater the 
ability of the photomultiplier to dis- 
criminate between pulses of nearly 
equal height. 

The pulse-height resolution para- 
meter is often measured by using 
the 662-keV photon from an iso- 
tope of cesium, Cs 137 , and a cylin- 
drical thallium-activated sodium- 
iodide scintillator. The Cs 137 source 
is placed in direct contact with the 
metal end of the scintillator pack- 
age and the faceplate end of the 
crystal is coupled to the tube by 
means of an optical coupling fluid. 
The load across which the pulse 
height is measured may consist of a 
100,000-ohm resistor and a capaci- 
tance of 100 picofarads in parallel. 
This combination provides a 10- 
microsecond time constant. 

Peak-to- Valley Ratio. In many 
low-energy scintillation-counting ap- 
plications, noise as well as resolution 



Basic Performance Characteristics 



47 



becomes important. A parameter 
which provides an indication of 
photomultiplier usefulness in resolv- 
ing low-energy X-rays is the pulse- 
height spectrum obtained for an 
Fe 55 radioactive isotope and shown 
in Fig. 40. This parameter is mea- 
sured by using an NaI(Tl) scintilla- 
tor with a thin beryllium window 
and an Fe 55 source. The decay of 
this isotope of iron results in a 5.9- 
keV X-ray. From the resulting spec- 
trum a peak-to-valley ratio and a 
pulse-height resolution can be cal- 
culated. 



Fe s5 S0URCE, ACTIVITY UCURIE. 
SCINTILLATOR: HARSHAW, TYPE HG 0.005 

8ERYLLIUM WINDOW, Nal (T/), 7/8 01 A. , 

0.040 THICK. 
CATH0DE-T0-DYN0DE No.l VOLTS • 660 
DYNODE No.l-TO-DYNODE No. 2 VOLTS = 108 
DYNODENo.2-TO-DYNODENo.3VOLTS = l5l 
EACH SUCCEEDING DYN0DE-STA6E VOLTS = 108 
ANODE-TO-CATHODE V0LTS=20O0 
FOCUSING ELECTRODE IS CONNECTED TO 

DYNODE No.l POTENTIAL. 
ELECTRON MULTIPLIER SHIELD IS CONNECTED 

TO DYNODE No.5 POTENTIAL 



10= 



1 — i — i — i — i — I — i — i i rz 




PEAK , 
VALLEY PEAK VALLEY 



I I ♦ I I I h I I I I I 



2 4 6 8 10 

PULSE HEIGHT— KeV 



Fig. 40 — Typical pulse-height spec- 
trum obtained for an Fe" radioactive 
isotope. 



The peak-to-valley ratio of the 
measured spectrum is a measure of 
the extent to which tube noise ob- 
scures the observation of low-energy 
gamma rays. However, at the larger 
values of peak-to-valley ratio (30 
to 50), the properties of the crystal 
become dominant. 

Afterpulsing. Afterpulses, which 
may be observed when photomulti- 
pliers are used to detect very short 
light flashes as in scintillation count- 
ing or in detecting short laser pulses, 
are identified as minor secondary 
pulses that follow a main anode- 
current pulse. There are two general 
types of afterpulses; both are char- 
acterized by their time of occurrence 
in relation to the main pulse. The 
first type results from light feedback 
from the area of the anode, or pos- 
sibly certain dynodes, to the photo- 
cathode; the intensity of the light is 
proportional to the tube currents. 
When this light feedback reaches the 
cathode, the afterpulse is produced. 
Afterpulses of this type, character- 
ized by a delay in the order of 40 
to 50 nanoseconds, may be a prob- 
lem in many older photomultipliers 
having open dynode structures. The 
time delay experienced with this type 
of afterpulse is equal to the total 
transit time of the signal through the 
photomultiplier plus the transit time 
of the light that is fed back. 

The second type of afterpulse has 
been shown to be the result of ioniza- 
tion of gas in the region between 
the cathode and first dynode. The 
time of occurrence of the afterpulse 
depends upon the type of residual 
gas involved and the mass of the gas 
ion, but usually ranges from 200 
nanoseconds to well over 1 micro- 
second after the main pulse. When 
the ion strikes the photocathode, sev- 
eral secondary electrons may be 
emitted; thus, the resulting after- 
pulse has an amplitude equal to sev- 



48 



RCA Photomultiplier Manual 



eral electron pulse-height equiva- 
lents. These pulses appear to be 
identical to the larger dark-current 
pulses, and it is suspected that many 
of the dark-current pulses are the 
result of photocathode bombardment 
by gas ions. 

Several gases, including N 2 + and 
H 2 +, are known to produce after- 
pulses. Each gas produces its own 
characteristic delay following the 
main pulse. The most troublesome, 
perhaps, is the afterpulse caused by 
the H 2 + ion; this afterpulse occurs 
approximately 300 nanoseconds after 
the main pulse. One source of hy- 
drogen in the tube is water vapor 
absorbed by the multiplier section 
before it is sealed to the exhaust sys- 
tem. Other gases which may cause 
afterpulsing may be present as a 
result of outgassing of the photomul- 
tiplier parts during processing or 
operation. Present photomultiplier 
processing techniques are designed 
to eliminate or at least to minimize 
the problem of afterpulsing. 



Time Response 

The commonly used methods of 
specifying time response involve 
frequency-domain and time-domain 
parameters. Frequency response is 
usually characterized in terms of the 
frequency at which the ac com- 
ponent of the output current de- 
creases to 3 dB below the low- 
frequency output level, and may be 
measured with an electro-optic modu- 
lator and a variable-frequency driving 
oscillator. Time-response measure- 
ments are made with small-spark 
sources, 2 light-emitting diodes, or 
mode-locked lasers 3 that approxi- 
mate delta-function light sources. In 
most measurements the entire photo- 
cathode is illuminated. 

The frequency response of a 
photomultiplier is related to the 
anode-pulse rise time, a parameter 
usually provided for each photo- 
multiplier type. The anode-pulse rise 
time of a tube, illustrated in Fig. 41, 
is affected primarily by the mechani- 




T 



CORRECTED FOR MEASURING 
EQUIP. RISE TIME (0.8ns) 



_L_L 



I l ll I I 



J L 



1000 



1200 1400 1600 1800 20QQ 2500 3000 3500 4000 4600 5000 
APPLIED VOLTAGE — VOLTS 



Fig. 41 — Anode-pulse rise times as a junction of anode-to-cathode applied 
voltage for a number of photomultipliers. 



Basic Performance Characteristics 



49 



cal configuration and the operating 
values of a device and is specified as 
the time for the current to increase 
from 10 to 90 per cent of maximum 
anode-pulse height when the tube is 
illuminated with a delta-function 
light source. 

Frequency response is also limited 
by the transit-time spread. This 
spread is the time interval between 
the half -amplitude points of the out- 
put pulse at the anode terminal, as 
shown in Fig. 42. Transit-time spread 
is seldom measured directly because 
it is difficult to determine whether 
the fall time of the pulse results from 
the fall time of the light source or 
the tube. Figs. 41 and 42 indicate 
that transit and rise times are re- 
duced as over-all applied voltage is 
increased. Tubes utilizing focus 
structures of either the linear or the 
circular-cage type exhibit better 
time-resolution characteristics than 
tubes of the venetian-blind type; in 
many high-speed applications this 
difference is of major importance. 



STABILITY 

The operating stability of a photo- 
multiplier depends on the magnitude 
of the average anode current; when 
stability is of prime importance, the 
use of average anode currents of 1 
microampere or less is recommended. 
Additional considerations affecting 
stability are discussed in the subse- 
quent paragraphs. 

Fatigue 

Tube fatigue or loss of anode sensi- 
ti"ity is a function of output-current 
level, dynode materials, and previous 
operating history. The amount of 
average current that a given photo- 
multiplier can withstand varies 
widely, even among tubes of the 
same type; consequently, only typi- 
cal patterns of fatigue may be cited. 

The sensitivity changes that are 
a direct function of large currents 
imposed for great lengths of time are 
thought to be the result of erosion of 
the cesium from the dynode surfaces 




|000 1200 1400 1600 1800 gQQQ 2500 3,300 3500 4OO0 45OO 5O00 
APPLIED VOLTAGE — VOLTS 

Pig. 42 — Transit-time spreads as a function of supply voltage for a number 

of photomultipliers. 



50 

during periods of heavy electron 
bombardment, and the subsequent 
deposition of the cesium on other 
areas within the photomultiplier. 
Sensitivity losses of this type, illus- 
trated in Fig. 43 for a 1P21 operated 
at an output current of 100 micro- 
amperes, may be reversed during 
periods of non-operation when the 
cesium may again return to the dy- 
node surfaces. This process of return 



RCA Photomultiplier Manual 




NO LIGHT ON TUBE 



100 200 300 400 
TIME— MINUTES 



500 



Fig. 43 — Short-time fatigue and re- 
covery characteristics of a typical 1P21 
operating at II volts per stage and with 
a light source adjusted to give 100 
microamperes initial anode current. At 
the end of 100 minutes the light is 
turned off and the tube allowed to re- 
cover sensitivity. Tubes recover ap- 
proximately as shown, whether the 
voltage is on or off. 

may be accelerated by heating the 
photomultiplier during periods of 
non-operation to a temperature 
within the maximum temperature 
rating of the tube; heating above the 
maximum rating may cause a perma- 
nent loss of sensitivity. 

Sensitivity losses for a given oper- 
ating current normally occur rather 
rapidly during initial operation and 
at a much slower rate after the tube 
has been in use for some time. Fig. 
44 shows this type of behavior for a 



100 




200 300 400 
TIME— HOURS 



500 



Fig. 44 — Typical sensitivity loss for 
a 1P21 operating at 100 volts per stage 
for a period of 500 hours. Initial anode 
current is 100 microamperes and is re- 
adjusted to this operating value at 48, 
168, and 360 hours. 

1P21 having CsSb dynodes, operat- 
ing at an output current of 100 
microamperes. Tubes operated at 
lower current levels, of the order of 
10 microamperes or less, experience 
less fatigue than those operated at 
higher currents, and, in fact, may 
actually recover from high-current 
operation during periods of low-cur- 
rent operation. 

Fatigue rates are also affected by 
the type of dynode materials used in 
a tube. Copper beryllium or silver 
magnesium dynodes are generally 
more stable at high operating cur- 
rents than the cesium antimony 
types. The sensitivity for tubes utiliz- 
ing these dynodes very often in- 
creases during initial hours of 
operation, after which a very gradual 
decrease takes place, as illustrated 
in Fig. 45. 

Photocathode Fatigue 

A photomultiplier with a multi- 
alkali photocathode (S-20 spectral 
response) tends to decrease in sensi- 



Basic Performance Characteristics 



51 




100 



400 



500 



200 300 
TIME — HOURS 

Fig. 45 — Typical sensitivity variation 
on life for a photomultiplier having 
silver-magnesium dynodes. Initial anode 
current was 2 milliamperes and was re- 
adjusted to this operating value at 48, 
168, and 360 hours. 

tivity upon extended exposure to 
high ambient room lighting. The 
mechanism by which this sensitivity 
loss occurs is not clearly understood. 
The decreased cathode sensitivity oc- 
curs primarily in the red portion of 
the spectrum and is usually perma- 
nent. It is interesting to note that 
photocathodes of the extended red 
multi-alkali (ERMA) type appar- 
ently do not exhibit this loss. 

The Ag-O-Cs photocathode (S-l 
spectral response) also suffers a de- 
crease in sensitivity, particularly dur- 
ing operation, when exposed to high 
radiant-energy levels normally not 
harmful to other types of photo- 
cathode materials. The decreased 
sensitivity occurs primarily in the 
infrared portion of the spectrum. 



Sensitivity may decrease quite rapidly 
upon application of light and volt- 
age during the first few minutes of 
operation. With a few hours of con- 
tinued operation, the sensitivity usu- 
ally increases until it has regained 
its initial value. If the source of the 
high radiant energy is removed from 
the tube, the sensitivity also tends to 
return to its initial value. 

"Hysteresis" 

Many photomultipliers exhibit a 
temporary instability in anode cur- 
rent and change in anode sensitivity 
for several seconds after voltage and 
light are applied. This instability, 
sometimes called hysteresis because 
of cyclic behavior, may be caused 
by electrons striking and charging 
the dynode support spacers and thus 
slightly changing the electron optics 
within the tube. Sensitivity may 
overshoot or undershoot a few per 
cent before reaching a stable value. 
The time to reach a stable value is 
related to the resistance of the in- 
sulator, its surface capacitance, and 
the local photomultiplier current. 
This instability may be a problem in 
applications such as photometry 
where a photomultiplier is used in 
a constant-anode-current mode by 
varying the photomultiplier voltage 
as the light input changes. 

Hysteresis has been eliminated in 
many tubes by coating the dynode 
spacers with a conductive material 
and maintaining them at cathode po- 
tential. Tubes treated by this method 
assume final sensitivity values almost 
immediately upon application of 
light and voltage. 

Counting Stability 

In scintillation counting it is par- 
ticularly important that the photo- 
multiplier have very good stability. 



52 



RCA Photomultiplier Manual 



There are two types of gain stability 
tests which have been used to evalu- 
ate photomultipliers for this applica- 
tion: (1) a test of long-term drift in 
pulse-height amplitude measured at 
a constant counting rate; and (2) a 
measure of short-term pulse-height 
amplitude shift with change in count- 
ing rate. 

In the time stability test, a pulse- 
height analyzer, a Cs 137 source, and 
a Nal(Tl) crystal are employed to 
measure the pulse height. The Cs 137 
source is located along the major 
axis of the tube and crystal so that 
a count-rate of 1000 counts per sec- 
ond is obtained. The entire system is 
allowed to warm up under operating 
conditions for a period of one-half 
to one hour before readings are re- 
corded. Following this period of 
stabilization, the pulse height is re- 
corded at one-hour intervals for a 
period of 16 hours. The drift rate in 
per cent is then calculated as the 
mean gain deviation (MGD) of the 
series of pulse-height measurements, 
as follows: 



2 Ip-piI 

MGD= l -=i 



100 
P 



where p is the mean pulse height, 
Pi is the pulse height at the i th read- 
ing, and n is the total number of 
readings. Typical maximum mean- 
gain-deviation values for photomul- 
tipliers with high-stability CuBe 
dynodes are usually less than 1 per 
cent when measured under the con- 
ditions specified above. Gain stability 
becomes particularly important when 
photopeaks produced by nuclear dis- 
integrations of nearly equal energy 
are being differentiated. 

In the count-rate stability test, the 
photomultiplier is first operated at 



10,000 counts per second. The photo- 
peak counting rate is then decreased 
to 1000 counts per second by increas- 
ing the source-to-crystal distance. 
The photopeak position is measured 
and compared with the last measure- 
ment made at a counting rate of 
10,000 per second. The count-rate 
stability is expressed as the percent- 
age gain shift for the count-rate 
change. It should be noted that 
count-rate stability is related to the 
hysteresis effect discussed above. 
Photomultipliers designed for count- 
ing stability may be expected to have 
a value of no greater than 1 per cent 
gain shift as measured by this count- 
rate stability test. 



Life Expectancy 

The life expectancy of a photo- 
multiplier, although related to fa- 
tigue, is very difficult to predict. 
Most photomultipliers will function 
satisfactorily through several thou- 
sand hours of conservative operation 
and proportionally less as the severity 
of operation increases. Although 
photomultipliers do not have ele- 
ments which "burn out" as in the 
case of a filament in a vacuum tube, 
photomultiplier dynodes do lose 
sensitivity with operation; however, 
as pointed out earlier, this loss oc- 
curs at an extremely low rate and. 
even tends to recover during con- 
servative operation. 

Factors which are known to af- 
fect life adversely are high-current 
operation, excessive-voltage opera- 
tion, high photocathode illumination, 
and high temperature. 

Operation of photomultipliers in 
regions of intense nuclear radiation 
or X-rays may result in an increase 
in noise and dark current as a re- 
sult of fluorescence and scintillation 
within the glass portions of the tubes. 



Basic Performance Characteristics 



53 



Continued exposure may cause dark- 
ening of the glass and a resultant 
reduction in transmission capability. 
Photomultipliers having glass en- 
velopes should not be operated in 
helium environments because helium 
can permeate the tube envelope and 
lead to eventual tube destruction. 

Extremely high illumination fo- 
cused onto a spot on the photo- 
cathode actually destroys a portion 
of the photocathode. It is usually 
better to defocus such a light source 
and utilize a larger area of the photo- 
cathode. 

Loss of photocathode sensitivity 
may sometimes result from the prox- 
imity of a high-potential gradient in 
the region of the photocathode. Such 
a gradient may result from applica- 
tions in which the photomultiplier 
is operated with the anode near 
ground potential. Metal fixtures, 
clamps, or shields at ground poten- 
tial may provide such a gradient 
through the glass near the photo- 
cathode either by proximity or direct 
contact. The result may be increased 
noise output and a deterioration of 
the photocathode through an elec- 
trolysis process through the glass. 
The destructive process may take a 
few hours or several hundred hours 
depending upon the material in con- 
tact with the envelope and the 
amount of potential difference. The 
deterioration process is greatly ac- 
celerated as the temperature is in- 
creased above normal ambients. 

LINEARITY 

The anode currents produced by 
photomultipliers are proportional to 
the level of incident radiation over 
a wide range of values. A linearity 
plot over a wide range of light level 
is shown in Fig. 46 for a type 931A. 
The limit of linearity occurs when 
space charge begins to form. Space- 




icr"' icr lz lo' 10 io-° io -6 io-* kt* \ 

LIGHT FLUX — LUMENS 

Fig. 46 — Range of anode-current line- 
arity as a function of light flux for a 
931A photomultiplier. 

charge-limiting effects usually occur 
in the space between the last two 
dynodes. The voltage gradient be- 
tween anode and last dynode is usu- 
ally much higher than between dy- 
nodes and, therefore, results in a 
limitation at the previous stage, even 
though the current is less. The maxi- 
mum output current, at the onset of 
space charge, is proportional to the 
3/2 power of the voltage gradient in 
the critical dynode region. By use 
of an unbalanced dynode-voltage dis- 
tribution increasing the interstage 
voltages near the output end of the 
multiplier, it is possible to increase 
the linear range of output current. 

Some photomultipliers used in ap- 
plications requiring high output pulse 
current use an accelerating grid be- 
tween the last dynode and the anode 
to reduce the effects of space-charge 
limiting. The potential of such a grid 
is usually between that of the last 
and the next-to-last dynode and is 
adjusted by observing and maximiz- 
ing the value of the anode output 
current. 

Another factor that may limit 
anode-output-current linearity is 
cathode resistivity; cathode resistiv- 
ity is a problem only in tubes with 
semitransparent photocathodes, par- 
ticularly of the CsSb or bialkali type. 



54 



RCA Photomultiplier Manual 



Individual tube bulletins and data 
listed in the Technical Data section 
of this manual give values of the 
peak linear and the peak saturated 
pulse currents. The limits of linear 
operation are determined, of course, 
by the particular geometry of the 
output stages and by the applied 
voltage. 

Peak linear and saturation cur- 
rents are measured by both dc and 
pulsed methods. One common 
method makes use of a cathode-ray 
tube with a P15 or P16 phosphor. 
The grid is double-pulsed with pulses 
of unequal amplitudes but fixed 
amplitude ratio. As the amplitude of 
the two pulses is increased, a point 
is observed at which the amplitude 
of the larger of the anode pulses does 
not increase in the same proportion 
as the smaller pulse. At this point 
the tube is assumed to become non- 
linear. The current value at this 
point is then measured by means of 
an oscilloscope and load resistor. 
The maximum saturation current is 
found when a further increase in 
radiation level yields no further in- 
crease in output. 

USE OF REFLECTED LIGHT 
AT THE PHOTOCATHODE 





TYPE 8054 






SUPPLY VOLTS 


= 1000 


IC S 


~ 1 1 


1111 = 


6 


~ 




4 


- 


^| / = 






^ \' — 


2 
l0 8 


— 


K? : 


;_. :^»^ 


>- - 


— 


^^w -— 


\- 6 










\ E 


> 4 


— 


t 


— 


V—««85» _ 


in 
Z 2 


- 


V 


*'°8 




W-8 = 0° = 

v - 


< 8 
Q 6 




IT 4 




UJ 

> 




\\ - 


(- 2 






< 
_J 




\\ 


UJ 






IT 10 




V = 


8 




6 


— 


V = 








4 


— 


' _ 


2 

1 


1 1 


1 1 1 1 



400 500 600 700 

WAVELENGTH - NANOMETERS 

Fig. 47 — Spectral-response character- 
istics for a CsSb transmission-type 
photocathode for two different incident 
angles of radiation. The increased red 
response for the large angle of incidence 
may be explained in part by the increase 
in absorption caused by the longer path. 



Because, in general, photocathodes 
absorb less red light than blue, the 
spectral response of the photocathode 
is somewhat sensitive to the angle of 
incidence of light on the photo- 
cathode. There is an increase in red 
sensitivity at larger angles of inci- 
dence, as shown in Fig. 47. It is 
possible to take advantage of this 
characteristic of photocathodes to 
enhance the photoemission signifi- 
cantly. 

If a prism is optically coupled to 
the faceplate of a transparent photo- 
cathode, the angle of the incident 
radiation may become such that 



total internal reflection occurs at 
both the photocathode-to-vacuum 
interface and the glass-to-air inter- 
face. Several passes of the light 
through the photocathode become 
possible and thus increase the prob- 
ability of producing photoemission. 
For total internal reflection to oc- 
cur, the minimum angle at which 
light may impinge upon the face- 
plate is given by arcsine (1/n), 
where n is the refractive index of 
the photomultiplier window material. 
The greatest enhancement pro- 
duced by the total internal-reflection 
method occurs in the red region of 



Basic Performance Characteristics 



55 



the photomultiplier spectral re- 
sponse; gains greater than 5 have 
been achieved at 700 nanometers in 
tubes having an S-20 spectral re- 
sponse. 

OUTPUT-CURRENT CONTROL 
AND SPATIAL UNIFORMITY 

Many photomultipliers are equipped 
with a focusing electrode, or grid, 
between the photocathode and the 
first dynode to provide optimum 
collection of the photoelectrons 
emitted from the photocathode. The 
focusing-grid voltage is usually set 
at the poiht at which maximum 
anode output current is obtained. In 
some applications, spatial uniformity, 
i.e., the variation of anode current 
with position of photocathode il- 
lumination, may be more important 
than maximizing output current. In 
such cases, however, the final adjust- 
ment of the focusing-grid potential 
should not differ significantly from 
the adjustment that provides opti- 
mized collection efficiency. Fig. 48 
shows a typical focusing-electrode 
characteristic. 

Spatial uniformity is determined 
by photocathode uniformity and by 
the uniformity of collection effi- 
ciency, i.e., the proportion of the 
emitted photoelectrons which strike 
a useful area on the first dynode. 

Photomultipliers with venetian- 
blind structures usually exhibit the 
best spatial uniformity because they 
have a large aperture area, and thus 
a relatively large first-dynode area. 
Modern high-speed tubes with fo- 
cused structures designed specifically 
for scintillation-counting applications 
also provide excellent spatial uni- 
formity; the collection efficiency in 
these tubes approaches unity. 

Cathode uniformity is determined 
by the uniformity of the deposition 
of the photocathode material. In 



FOCUSING-ELECTRODE VOLTAGE IS VARIED 
BY ADJUSTMENT OF POTENTIOMETER 
CONNECTED BETWEEN DYNODE No. I AND 
CATHODE. 




20 40 60 80 100 

FOCUSING ELECTRODE VOLTAGE — 
% OF DYNODE No. I -TO- CATHODE VOLTS 

Fig. 48 — A typical focusing-electrode 
characteristic. 



general, large tubes having relatively 
large distances between the activat- 
ing elements and the photocathode 
exhibit the best cathode uniformity. 
In these tubes, the activating ele- 
ments, of which there may be sev- 
eral, approach point sources. 



REFERENCES 



1. M. Lontie-Bailliez and A. Mes- 
sen, "L'Influence de la Tempera- 
ture sur les Photomultiplicateurs," 

''Annates de la Societe Scientifique 
de Bruxelles, Vol. 73, Series 1, 
p. 390 (1959) 

2. A. Kerns, A. Kirsten, and C. Cox, 
"Generator of Nanosecond Light 
Pulses for Phototube Testing," 
Rev. Sci. Instr., Vol. 30, No. 1, 
p. 31 (1959) 

3. A. J. DeMaria, W. H. Glenn, Jr., 
M. J. Brienza, and M. E. Mack, 
"Picosecond Laser Pulses," Proc. 
IEEE, Vol. 57, No. 1, p. 2 (1969) 



56 



Statistical Fluctuation and Noise 



IN this section, some of the sources 
of noise and output fluctuations 
in photomultipliers are examined. 
These sources may be roughly di- 
vided into two classes: (1) those re- 
sulting from the processes by which 
the photomultiplier operates, and 
(2) those resulting from the way in 
which a tube is designed and manu- 
factured and the conditions under 
which it is operated. The first class 
is more fundamental because it com- 
prises the statistical processes of 
photoelectric emission and electron 
multiplication through secondary 
emission. Most of the discussion of 
class 1 sources deals with these 
processes and their effect on photo- 
multiplier output. Class 2 sources 
include thermionic emission from the 
photocathode, spurious gas ionization 
within the tube, light feedback from 
the output stages to the photocathode, 
and random emission caused by the 
natural radioactivity of the various 
materials within the tube. 

A photomultiplier is so sensitive 
a radiation detector that it can oper- 
ate at the lower limit of detectability 
where output pulses are initiated by 
single photoelectrons from the photo- 
cathode. A count of the number of 
output pulses in a given time inter- 
val provides a measure of the rate 
at which photons strike the photo- 
cathode. This mode of operation is 
called the photon-counting mode; it 



is at this lower limit of photomulti- 
plier operation that noise sources 
become most important. Therefore, 
most of the discussion in this sec- 
tion centers upon the effect of the 
noise sources on tube output in this 
mode. 

PHOTON NOISE 

If the photon flux impinging upon 
the photocathode originates from a 
thermal broad-band source, the pho- 
tons can be assumed to be emitted 
at random time intervals. Under 
such conditions, the probability that 
n photons will strike the photo- 
cathode in a time interval t is given 
by a Poisson distribution, as follows: 



P(n,r) = 



(I P r) n 
n! 



exp (-I p t) (4) 



where I p is the average photon-arrival 
rate. The average value for n and the 
variance in n, o- p 2 , can be obtained 
directly from P(n, t): 



n = I D 



o-p = I p t = n 



(5) 
(6) 



The signal-to-noise ratio n/tr p is 
given by 



SNR = (I p r)* 



(7) 



Statistical Fluctuation and Noise 



57 



Thus the signal-to-noise r atio o f the 
photon flux increases as ylpr. 

There are a few cases in which 
the fluctuation in the photon flux 
has an additional term. 1 In broad- 
band thermal sources, however, Eq. 
(6) yields the proper expression for 
the noise in the input photon flux. 

NOISE CONTRIBUTIONS FROM 
THE PHOTOCATHODE 

The physics of photoemission and 
photocathodes is discussed in the 
section on Photoemission and Sec- 
ondary Emission. In the following 
discussion it is assumed that the time 
between the absorption of a photon 
and the subsequent emission of an 
electron, when emission occurs, is 
short (about 10 -12 second). In ad- 
dition, it is assumed that all the 
statistical processes of absorption, 
electron transport within the photo- 
cathode, and photoemission can be 
described and characterized by one 
number, the quantum efficiency tj. 
For a given photocathode, 77 is a 
function of the photon wavelength, 
as explained in the section on Photo- 
emission and Secondary Emission. 

A simple model of photoemission 
will aid in understanding the noise 
contributions of the photocathode. 
For each photon that strikes the 
photocathode, an electron is emitted 
with probability tj. Optical-reflection 
effects at the various cathode inter- 
faces are accounted for in the value 
of 17. The chance that no electron is 
emitted is 1— tj. 

The statistics for this process can 
be found through the use of the gen- 
erating function Q P .cX s ) derived in 
Appendix A at the end of this sec- 
tion. In the simplified model of the 
photocathode this function is given 
by 

Qp.c.(s>= (1-17) + si? (8) 



From the expressions developed in 
Appendix A for the average and vari- 
ance, the average output n and the 
variance <r 2 are given by 



n = ■q 
= r,(l-r,) 



(9) 
(10) 



The discussion of these expressions 
is best understood by imagining a 
steady stream of photons impinging 
upon the photocathode. The photons 
are equally spaced in time and ar- 
rive at a rate of I p photons per sec- 
ond. In an interval t (where t is 
much larger than 1/I P ), the number 
of photons N striking the photo- 
cathode is equal to I pT . Because by 
hypothesis I p is constant, the number 
N does not fluctuate. In a time t, the 
average number of photoelectrons 
n emitted from the photocathode is 
equal to tjN. The fluctuations in 
photoelectron number can be com- 
puted by use of Eq. (84) in Appen- 
dix A for the total variance for two 
statistical processes in series. Be- 
cause the photon flux does not fluc- 
tuate, the total variance in the 
photoelectron number is given by 



A.: -N(l-u)u 



(11) 



Eq. (11) can be derived from Eq. 
(84) in Appendix A with the follow- 
ing substitutions: 

n A = N, n B = 71, cr A 2 = 0, and 
o- B 2 = 77(1—17). The signal-to-noise 
ratio of the photon flux is infinite 
because it does not fluctuate; how- 
ever, the photoelection number has 
a signal-to-noise ratio given by 



SNR, 



-[^J 



(12) 



Thus, for quantum efficiencies less 
than unity, the statistics of the photo- 
cathode result in a finite signal-to- 



58 



RCA Photomultiplier Manual 



noise ratio for the photoelectron 
number. As shown below, this state- 
ment is valid even when the input 
photon beam fluctuates. 

Fig. 49 shows a curve of the signal- 
to-noise ratio of the photoelectron 
number produced by the hypothetical 
noiseless photon flux. When ij = 1, 
the photocathode is a perfect con- 
verter and no noise is impressed on 
the photoelectron number. When tj 
is less than 1, the signal-to-noise 
ratio decreases with decreasing 77 
until, for values much smaller than 
1, the signal-to-noise ratio becomes 
proportional to the square root of 77. 



and 



SNR P = |5p]* 



(13) 



(14) 



where n p is the average number of 
photons arriving in a time interval 
T. The average photoelectron num- 
ber and its variance are calculated 
from Eqs. (81) and (84), respec- 
tively, in Appendix A. The average 
number of photoelectrons and the 
variance in the photoelectron number 
are stated, respectively, as 

rlp.e. = ri n p (15) 

and <r 2 = n p r, (1 -ij) + 17 2 n p 



co 6 - 



o 
o 




or 



1"p 



(16) 



0.2 0.4 0.6 0.8 1.0 

PHOTOCATHODE QUANTUM EFFICIENCY (OE) 

Fig. 49 — The photoelectron signal-to- 
noise ratio resulting from a noiseless 
photon flux impinging upon the 
photocathode. 



In practice, the signal-to-noise 
ratio of the input photon flux is 
never infinite; the flux always con- 
tains some noise. In most sources, 
the signal-to-noise ratio of the photon 
flux is given by Eq. (7) and the 
variance of the photon beam is given 
by 



It should be noted that the statisti- 
cal conversion process within the 
photocathode has not altered the 
functional form of the variance; it 
is still proportional to the average 
number of particles. The photoelec- 
trons appear to emanate from a ran- 
dom source of electrons with average 
value t)5 p . It is as though the aver- 
age photon number were reduced 
by a factor 77 in the conversion 
process, a result that depends on 
the variance of the photon flux be- 
ing equal to n p . 

The photoelectron signal-to-noise 
ratio is 



SNR„ 



[i»5pl* 



(17) 



A quantum efficiency of 40 per cent 
reduces the signal-to-noise ratio of 
the photoelectron flux to about 63 
per cent of that of the photon flux. 

It is important to realize that the 
degradation in signal-to-noise ratio 
as a result of 17 being less than unity 
is irreversible in that no amount of 
noise-free amplification can improve 
the photoelectron signal-to-noise 
ratio. 



Statistical Fluctuation and Noise 



59 



In some applications, multiphoton 
pulses form the input signal. In these 
applications, integral numbers of 
photoelectrons are emitted from the 
photocathode within a time that is 
short with respect to the resolution 
capabilities of the photomultiplier as 
a result of a multiphoton input 
pulse. Examples of this type of in- 
put can be found in radioactive 
tracer scintillations, such as those 
observed in tritium and carbon spec- 
troscopy. 

If, with -q equal to 1, the input 
signal consists of a steady train of 
photon pulses widely spaced in time, 
each pulse consisting of m photons, 
a steady train of photoelectron pulses 
would result, each consisting of m 
electrons. For values of r) less than 
1, the number of electrons in each 
pulse, r, can vary from to m 
(0 < r < m). The probability that 
r electrons are emitted is obtained 
through the use of the multiple-input- 
particle generation function de- 
scribed by Eq. (86) in Appendix A. 
For m-photon input pulses, the gen- 
erating function Q m (s) is given by 
[Q(s)] m , where Q(s) is the single- 
particle generating function. Thus, 
the function Q m (s) is given by 

Q m (s) = [(l-„) + „s] m (18) 

The probability that r photoelectrons 
will be emitted is given by 



Prfo) 



i a r Q m (s) 

(r)! 6s r 



or 



s = (19) 
m! 



M«>-*'(i-*~[hc^] 



(20) 



Eq. (20) is just the coefficient of s r 
in the expression for Q m (s). P r (i7) 
is composed of the chance of r suc- 
cessful photoemissions (rj r )» the 
chance of <m— r) failures to photo- 



emit [(1— Tj) m_r ], and the binomial 

coefficient — - — : — - which de- 
|_r! (m— r)!J 

scribes the number of ways such an 
output can occur among the m- 
photon inputs. 

Figs. 50 and 51 show the probabil- 
ity distribution for photoemission 
from the photocathode for a train of 
4-photon input pulses and for two 
different values of rj. The spectrum 
in Fig. 50 is computed with 



0.4 



0.3 



- 0.2 



0.1 



12 3 4 

No. OF PH0TOELECTRONS/PULSE — r 



Fig. 50 — Photoelectron output pulse 

spectra resulting from a flux of 4-photon 

input pulses computed with >j = 0.4. 

■q = 0.4 and that of Fig. 51 with 
7) = 0.01. The difference is strik- 
ing. With 7] — 0.4, there is no photo- 
emission only 13 per cent of the time; 
most of the photoemission is di- 
vided between 1- and (2-, 3-, or 4-) 
photoelectron pulses in the ratio of 
13 to 20. In sharp contrast, when 
7j = 0.01, photoemission does not 
occur 96 per cent of the time. Of 
the times when photoelectrons are 
emitted, single-electron pulses occur 
70 times more often than any others; 
3- and 4-photoelectron pulses almost 
never occur. It should be clear that 
in multiphoton-pulse spectroscopy it 
is important that ij be as high as 
possible. 



60 



RCA Photomultiplier Manual 



1.0 



17=0.01 



4X10 z 6XI0-" 4XIO" 6 lO" 1 
1 i t i_ 



12 3 4 

No. OF PHOTOELECTRONS/ PU LSE — r 



Fig. 51 — Photoelectron output pulse 

resulting from a flux of 4-photon input 

pulses computed with -q = 0.01. 

As in the case of a single-photon 
pulse train, the average photoelectron 
number and the variance in that 
number can be found from Q m (s) 
by use of Eqs. (87) and (88) of Ap- 
pendix A. If a is the average rate of 
randomly emitted m-photon pulses, 
then the average number of photo- 
electrons emitted in a time r (where 
t is much greater than 1/a) is given 
by 

TTp.e. = rj m(ar) (21) 

and the variance is given by 

< e . = V m(a T ) (22) 

The signal-to-noise ratio is given by 

SNR p .e. = [»? m(ai-)]* (23) 

The input signal-to-noise ratio 
(mar) 1/2 is degraded by a factor 
of 0.63 for tj = 0.4 and by a fac- 
tor of 0.1 for 7] = 0.01. 

The effect of the imperfect conver- 
sion of photons from a purely ran- 
dom source into photoelectrons 
within the cathode is a reduction in 
the input signal-to-noise ratio by a 
factor equal to the square root of 
r). Deviations from this result indi- 
cate that the photon source may not 



be purely random. Such effects have 
been observed but are small in or- 
dinary thermal sources. 1 

Another source of photocathode 
noise is the thermionic emission of 
single electrons. The strength of the 
emission varies with photocathode 
type. In the bialkali cathode, for ex- 
ample, thermionic emission is vir- 
tually absent; electron emission exists 
in the absence of a photo signal, but 
it is not temperature-dependent as 
is thermionic dark emission, and it 
probably arises from a different 
source. S-l photocathodes, on the 
other hand, exhibit relatively large 
dark currents as a result of thermionic 
emission; these currents can be elimi- 
nated to some extent, > however, by 
cooling the tube. 

Randomly emitted thermionic elec- 
trons add a term to the fluctuation 
in the photoelectron current pro- 
portional to their number. Because 
of their independence with respect to 
any usual signal current, the term 
adds to that of the signal noise in 
quadrature. That is, 



°p.e. = in p + n th 



(24) 



where n th is the average number of 
thermionic electrons emitted in a 
time r, and 7jn p is the average num- 
ber of photoelectrons emitted in the 
same time. 

NOISE CONTRIBUTIONS FROM 
THE MULTIPLIER CHAIN 

Although the amplification (multi- 
plication) of the photoelectron cur- 
rent in the dynode chain of a photo- 
multiplier is often referred to as 
noise-free, careful examination of the 
statistics of the gain mechanisms in- 
volved shows that this statement is 
not entirely correct. Approximately 
noise-free operation can be attained, 
however, with the use of proper elec- 



Statistical Fluctuation and Noise 



61 



tron optics and newly developed dy- 
node materials. In the following dis- 
cussion the statistical gain processes 
in the individual dynodes are ex- 
amined and then combined to yield 
the statistical properties of the entire 
multiplier chain. Much of the work 
presented was accomplished at a 
very early stage in photomultiplier 
history. 2 

Secondary Emission 

Photomultiplier dynodes provide 
gain through the process of second- 
ary emission, which was discussed in 
detail in the section on Photoemission 
and Secondary Emission. Most prac- 
tical dynodes consist of semiconduc- 
tors or of conducting substrates 
surfaced with a thin insulating film. 
An energetic primary electron pene- 
trates the surface and, ideally, ex- 
cites electrons from the valence band 
to the conduction band within the 
dynode material. These excited elec- 
trons drift and diffuse toward the 
dynode-vacuum interface where they 
escape with a certain probability into 
the vacuum as secondary electrons. 
For a given primary energy, it is pos- 
sible to obtain any number of sec- 
ondaries n 8 from zero to a maximum 
n s(max)- The maximum number is 
given by the quotient of the primary 
energy E p and the energy required 
to produce a hole-electron pair 
within the dynode e p , as follows: 



n 8 (mai) = E p /e p 



(25) 



Over many repeated measurements 
using primary electrons of the same 
energy, a truncated distribution is 
obtained for n s . A model that de- 
scribes the observed distributions 
from most practical dynodes is de- 
veloped in Appendix B at the end 
of this section. 



Statistics for a 
Series of Dynodes 

Using the formulas for the gain 
and fluctuation for a single dynode 
developed in Appendix A, the ex- 
pressions of Appendix B may be used 
to find the combined gain and vari- 
ances for the string of dynodes that 
make up a multiplier. It is assumed 
that one primary electron impinging 
on the first dynode releases, on the 
average, 6\ secondaries with variance 
o~i 2 . The output of the first dynode 
striking the second dynode produces 
an average gain at the second stage 
of m 2 and a variance of 0- 2 m2 . From 
Eq. (84) in Appendix A, m 2 and o-| m 
can be related to the individual dy- 
node statistics as follows: 



ni2 = 5i • 52 



(26) 



and 



Tm2 = 5 2 a 1 + Si a\ (27) 



where 8 2 ar| d or 2 2 are the average 
secondary emission and variance for 
the second dynode for a single in- 
put electron. Continuing in this man- 
ner, the gain and fluctuation from 
the third stage are given by 



ni3 = 8y 82-83 



(28) 



and 

<£i = «J $ «? + 81 of] + «i 82 4 (29) 

Eq. (29) can be rearranged to read 



'mS 



(30) 



The extension of Eqs. (29) and (30) 
to k stages is accomplished by add- 
ing terms to Eqs. (26) and (27) as 
follows: 



62 



RCA Photomultiplier Manual 



m k = 5i-5 2 5 k (31) 



^=^y + 



«** 



(8i-8f "Sk-i) 8k 



(32) 



Eq. (31) states the expected results: 
that the total average gain for a 
series of k dynodes is the product 
of the secondary-emission yields of 
the individual dynodes in the series. 
Eq. (32) shows that the relative con- 
tribution of any stage to the total 
fluctuation decreases with the prox- 
imity of the dynodes to the output 
end of the chain. The first stage con- 
tributes most to the total variance. 
The higher the first-stage gain, the 
less each subsequent stage contributes 
to the total variance. This property 
is an important feature of the new 
high-gain GaP first-dynode photo- 
multipliers. 

The signal-to-noise ratio for the 
multiplier chain is given by 



The noise added to the input signal 
is very small. It is in this sense that 
the multiplication chain is said to 
provide noise-free gain. 

Multiple-Particle Inputs 

The output-pulse distribution for 
multiple-particle inputs is obtained 
from the generating function for the 
multiplier chain. A multiplier chain 
consisting of k dynodes has a gen- 
erating function Q k (s) given by 

Qk(s) = Qt {Q 2 [ Q k (s)]} (35) 

The probability P k (n) of observing 
n electrons (for a one-electron in- 
put) at the output of a k-stage chain 
is derived from Eq. (74) in Appendix 
A, as follows: 



Pk(n) = n!-5 S " 



Qk(s) 



s = 



(36) 



The generating function for a k-stage 
multiplier chain for multiple-particle 
inputs is given by Eq. (86) of Ap- 
pendix A, as follows: 



mi^ _ Taj 
Omk \_S\ 



A. 

5i5 



\o 2 



+ 



4 



j] (33) 



(5i-5 2 - • -5k-i) 8 k 



For large first-stage gains, the mul- 
tiplier signal-to-noise ratio is high. 
Most of the noise contribution is 
from the first stage. If, in addition 
to a large gain, the first stage exhibits 
Poisson statistics, as explained in 
Appendix B, the signal-to-noise ratio 
becomes 



SNR k = 



m k 

ffmk 



Vli (34) 



Q k (s,m) = [Q k (s)] n 



(37) 



The probability of observing n output 
electrons from m input particles is 
given by 



p . . 1 dJQk (s)] n 
Pk (n,m) = — — 5— — 
n! ds n 







(38) 



When identical dynodes are used, the 
output distribution for single-elec- 
tron input pulses evolves toward a 
steady-state distribution after four or 
five stages, and exhibits little change 
thereafter. Fig. 52 shows some 
single-electron distributions 3 com- 
puted by use of the Polya statistics 
for each stage in the chain as ex- 
plained in Appendix B. As b ap- 



Statistical Fluctuation and Noise 



63 





\ „-b=I.O 


1 


1 1 ' 




0.8 






STAGE GAIN(^)»3 


- 


H 0.6 










z 

UJ 

o 
>- 


/ // b=04 






- 


t .4 

CD 
< 

CD 
O 
CC 
D. 

0.2 


~ll^a=0.Z 
lf^~~—- b = 


MEAN 

1 

1 




- 






1 ~^ =£aaB - as 


L 



PULSE HEIGHT 

Fig. 52 — ■ Computed single-electron distribution for a range of values of parameter 
b. Parameter b is defined in Appendix B. 

proaches 1, the distribution becomes structure are shown in Fig. 53. 4 The 

more sharply peaked. two curves relate to two different 

Computer values of multiple- structures that have the same dynode 

particle outputs for a two-stage as a first stage. The solid-line curve 



~1 1 - 

CURVE A 

STAGE I: M=5.0, b = 0.20 

STAGE 2: a» = 500, b»0.0l 

CURVE B 

STAGE I : /i = 5.0, b«0.20 

STAGE2: /i=500, bM.OO 




s 



I 2 3 4 5 6 7 8 9 10 II 12 

NUMBER OF SECONDARY ELECTRONS IN A PULSE FROM DYNODE SURFACE 



Fig. 53 — Theoretical pulse-height distribution. 



64 



RCA Photomultiplier Manual 



shows the output when the first stage 
is followed by a high-gain (/i = 500) 
second stage having nearly pure 
Poisson statistics (b = 0.01). The 
output peaks are sharply defined, and 
pulses up to a ten-electron input 
pulse are clearly resolvable. The 
dashed line describes the final out- 
put distribution when the first stage 
is followed by a high-gain fyi = 500) 
dynode having an exponential output 
distribution. Individual peaks are no 
longer discernible; the large variance 
associated with the exponential sta- 
tistics of the second stage eliminates 
all the structure in the output of 
the first stage. 

RCA has recently developed a 
high-gain gallium phosphide (GaP) 
dynode which, when used as the first 
stage in a conventional copper beryl- 
lium (CuBe) multiplier chain, greatly 
increases the pulse-height resolution 
of the photomultiplier. The high-gain 
first stage in a photomultiplier hav- 
ing multiple photoelectron events 
originating from the photocathode is 
similar to the case illustrated in Fig. 
53 for a multiplier where the high- 
gain second dynode amplifies the 
multiple pulses originating from the 
first dynode which in turn are ini- 
tiated by single electrons. Typical 




12 3 4 5 6 7 

PULSE HEIGHT -PHOTOELECTRON EQUI- 
VALENTS 

Fig. 54 — Typical photoelectron pulse- 
height spectrum for a photomultiplier 
having a GaP first dynode. 



gains for the new RCA gallium phos- 
phide dynodes are 30 to 45, and 
their statistics are nearly Poisson. 
Fig. 54 shows the multiple-particle 
pulse-height distribution for the 
tube, and Fig. 55 shows the pulse- 
height curves for a tritium scintilla- 
tion input for a conventional tube 
using the standard copper-beryllium 
first dynode along with that for a 
tube with a gallium phosphide first 
dynode. The increased resolution of 
the gallium phosphide dynode is 
clearly shown. 



-8850 (H3) 




CHANNEL No. 
(COMPRESSED SCALE) 

Fig. 55 — A comparison of the tritium 
scintillation pulse-height spectra ob- 
tained using a conventional photomul- 
tiplier having all CuBe dynodes and a 
photomultiplier having a GaP first stage. 

Note: The first photoelectron peak of 
the 8850 spectra includes dark 
noise from the photomultiplier, 
chemiluminescence and phos- 
phorescence from the vial and 
cocktail, and well as H° disinte- 
gration. 

FLUCTUATIONS IN THE 
TUBE AS A WHOLE 

In the previous sections the noise 
contributions from the photocathode 
and the multiplier chain were consid- 
ered; these results can be combined 
to obtain the signal-to-noise ratio for 
the photomultiplier as a whole. 

The average number of photoelec- 
trons from the photocathode in a 
time t is given by 



Statistical Fluctuation and Noise 



65 



n P .e. = i)*n p 
The variance is given by 



Tp.e. = T 5 P 



(39) 



(40) 



It is assumed that cr/ = n p or that 
the input flux of photons displays 
Poisson statistics. If the form for 
o- p 2 is not the result of a Poisson 
process, Eq. (84) in Appendix A must 
be used to obtain o- pe 2 . 

Using these expressions to describe 
the input to the photomultiplier 
chain, the average number of elec- 
trons collected at the anode can be 
stated as follows: 



n a = ipn p «mic 



(41) 



where m k is given in Eq. (31) and 
is the average gain of a k-stage multi- 
plier. The variance for the output 
electron stream is given by 

<r\ = m 2 k -rj-n p + irV^mk (42) 

where cr mk 2 is given by Eq. (32) and 
is the variance in the average gain 
of a k-stage multiplier chain. 

Eq. (42) can be rearranged as 
follows: 

<r*-i|npa&2+<rik) (43) 

For equal-gain stages described by 
Poisson statistics in the multiplier 
chain, Eq. (42) becomes 



"mk 



-(&[-*] '"' 



Neglecting the l/8 k term and sub- 
stituting the result and S k for m k 
in Eq. (43), 



High dynode gains imply that 
l/<8— 1) is much less than 1 and 
hence that 



o\ = „n p (*») 



(47) 



The signal-to-noise ratio at the anode 
is given by 



SNR a 



- = fo2p)* 
<r a 



(48) 



For high-gain dynodes exhibiting 
Poisson statistics, therefore, SNR a is 
essentially that of the photoelectrons, 
SNRp.e., as given in Eq. (17). 

In a photomultiplier in which the 
dynode gain is not high but still ex- 
hibits Poisson statistics, SNR a is 
given by 



SNR a = 



0?n p )» 



(49) 



As an example, with 8 = 4 the SNR a 
is decreased by a factor of 0.87 from 
its value for very large 8. Doubling 
8 to a value of 8 changes the deg- 
radation factor to 0.94. Further in- 
creases in 8 do not improve the 
SNR a very much. 

In the case of fully exponential 
dynode statistics, the variance for 
each dynode in a chain of identical 
dynodes is given by Eq. (94) in Ap- 
pendix B; i.e., 



8 2 + S 



(50) 



where 8 is the average gain per stage. 
The total anode fluctuation then be- 



comes 



In this case 



SNR a 



v* 5 >/(fO* 



(46) 



For large dynode gains, 

a\ m 2faHp) « 2k 



(52) 



66 



RCA Photomultiplier Manual 



Even for large dynode gains, ex- 
ponential statistics increase o- a 2 by a 
factor of 2, with the result that SNR a 
decreases by 1/^2. This drop in 
SNR a is accompanied by a severe 
loss in single- and multiple-electron 
pulse-height resolution, as shown in 
Fig. 53. To resolve single-photoelec- 
tron pulses, the multiplier chain must 
exhibit both high gain and good 
(i.e., Poisson) statistics. 

A significant improvement in 
SNR a results when the photocathode 
quantum efficiency T/ is increased. At 
present the peak value of tj for the 
S-20 cathode is 0.35; doubling 77 to 
0.7 would improve SNR a by a factor 
of 1.4. The ideal photomultiplier, for 
which 7) = 1, would have an SNR a 
equal to the square root of n p . The 
present value of 7) = 0.35 degrades 
the ideal SNR a by a factor of 0.59. 
Considerable improvement can be ex- 
pected with the development of 
photocathode materials of increased 
sensitivity. 

The application of these equations 
to present photomultipliers indicates 
that the available photocathode quan- 
tum efficiency is the principal degrad- 
ing influence on SNR a . Values of 77 
less than 1 also decrease the mul- 
tiple-photon input-pulse resolution of 
a photomultiplier. High dynode gains 
(8 greater than 6) do not significantly 
degrade the input signal-to-noise 
ratio of photoelectrons provided the 
dynode statistics are nearly Poisson. 



OTHER SOURCES OF NOISE IN 
PHOTOMULTIPLIER TUBES 

Within a photomultiplier there are 
sources of noise that are not asso- 
ciated directly with the processes of 
photoelectric conversion and elec- 
tron multiplication. These sources 
can, in general, be separated into two 
groups: (1) those that are not cor- 



related with and (2) those that are 
correlated with the signal pulse. 

GROUP 1 NOISE SOURCES 

Radioactivity within the 
Photomultiplier 

The materials used to fabricate 
the internal structure and the glass 
envelope of a photomultiplier may 
contain amounts of certain radio- 
active elements that decay and give 
off gamma rays or other high-energy 
particles. If one of these emitted 
particles strikes the photocathode or 
one of the first few dynodes, it will 
produce an anode dark pulse. The 
size of the anode pulse may be 
equivalent to one or more photoelec- 
trons emitted at the photocathode. 
These pulses are randomly emitted. 
In tube manufacture this type of 
emission is minimized through the 
careful selection of materials. 

Dark Pulses of Unknown Origin 

Dark pulses of unknown origin in 
a photomultiplier result primarily 
from single-electron pulses; a small 
fraction consists of 2- and 3-electron 
pulses. The rate of occurrence of 
these dark pulses varies among mul- 
tipliers; the lowest rate is about 150 
counts per minute for the new RCA- 
8855 series. These dark pulses are 
randomly emitted, but may be ac- 
companied by one or more of the 
noise effects correlated with the sig- 
nal pulse. The problem of the origin 
of unknown pulses is at present a 
very active field of research. 

GROUP 2 CORRELATED 
NOISE SOURCES 

Gas Ionization 

A gas atom or molecule within the 
photomultiplier may be ionized by a 



Statistical Fluctuation and Noise 



67 



photoelectron pulse. This ionization 
may occur at the first-dynode sur- 
face or in the vacuum between the 
photocathode and the first dynode. 
The positive ion thus created travels 
backward to the cathode where it 
may release one or more electrons 
from the photocathode. Because 
there is a time delay in the ion- 
emitted electron pulse equal to the 
time of flight of the ion to the photo- 
cathode, the resulting pulse, usually 
referred to as an afterpulse, occurs 
after the true signal pulse at the 
anode. Afterpulses are caused mainly 
by hydrogen ions and their occur- 
rence can be minimized in tube 
processing. 

Light Feedback 

Primary electrons produce photons 
as well as secondary electrons within 
the dynodes of the multiplier chain. 
Despite the low efficiency of this 
process, some of the emitted photons 
may eventually reach the photo- 
cathode and release additional elec- 
trons. A time delay is observed 
corresponding to the transit time for 
the regenerated electron pulse to 
reach the point of origin of the light. 
Depending upon the type of dynode 
multiplier cage, this time may be of 
the order of 20 nanoseconds. Most 
of the photons comprising the light 
feedback originate in the region of 
the last few dynodes or of the anode. 

If the voltage across the tube is 
increased, the dark-pulse rate also 
increases and usually produces some 
observable light near the anode re- 
gion, a fraction of which is fed back 
to the photocathode. The result of 
this positive feedback is that, at a 
certain voltage, the photomultiplier 
becomes unstable and allows the out- 
put dark-pulse rate to increase to an 
intolerably high level. The voltage 
at which this increase occurs is gen- 



erally above the recommended maxi- 
mum operating voltage. 

Not every signal pulse initiates an 
afterpulse. Therefore, coincidence 
techniques, using more than one 
photomultiplier, can be employed in 
some instances to eliminate this 
source of noise as well as the un- 
correlated sources discussed above. 

Analytically, the fluctuations re- 
sulting from Group 1 non-correlated 
random sources add in quadrature 
to those of the signal. Because they 
are random, the dark-pulse variances 
(Tap 2 are proportional to the aver- 
age dark-pulse rate n dp , as follows: 



"dp = n dp 



(53) 



The total anode variances are given 
by: 

"\ = (V 5 P + B dp) 

K i+ ^)] (M) 

where rjn p and n dp are the average 
single-photoelectron and the average 
single-dark-emitted electron numbers 
observed in a time t. The factor 
n dp sets a real limit on the anode 
signal-to-noise ratio. Again, in those 
instances where the incoming radia- 
tion signal comprises more than one 
photon, coincidence techniques can 
be employed to reduce the effect of 
randomly emitted electrons originat- 
ing at the cathode. 

Electrons may originate at dynodes 
well along in the multiplier chain. 
At the anode, these electrons appear 
as fractional-photoelectron pulses. 
Such pulses also result from inter- 
stage skipping, generally near the be- 
ginning of the chain. With good 
statistics in the chain, the fractional 
pulses may be discriminated against 
because the single-electron peak in 
the pulse-height spectrum stands out 
sharply. 



68 



RCA Photomultiplier Manual 



NOISE AND THE BANDWIDTH 
OF THE OBSERVATION 

At high counting rates, noise cal- 
culations are performed with the av- 
erage and variance of the photo- 
electron current rather than with 
individual photoelectron pulses. The 
expressions which have been de- 
veloped for signal-to-noise ratio by 
consideration of the average number 
of events in a time t and the variance 
from average may be converted to 
expressions of signal-to-noise ratio 
involving currents and bandwith, Af, 
by considering the reciprocal nature 
of the observation time and the band- 
width. 

Noise equivalent bandwidth Af 
may be defined 5 as follows: 



Af 






|H(jw)| 2 df (55) 



where H(jw) is the complex fre- 
quency transfer response of the cir- 
cuit, and A m is the maximum abso- 
lute value of H(jo)). The "circuit" 
in this case counts pulses in a time 
r. The network transfer function is 
the Laplace transform of the im- 
pulse response. The impulse response 
is a rectangular pulse of width r, 
like a camera shutter. For this case, 



H(j«) = ^ (1 - e-'~) (56) 
]<° 

and A m is found to be equal to r. 
The noise equivalent bandwidth is 
then readily found to be 



Af = l/2r. 



(57) 



It is of interest to compare this re- 
lation with the equivalent noise 
bandwidth for exponential impulse 
response as in an RC circuit; in this 
case 



Af = 1/4RC 



(58) 



From Eq. (7), the SNR for a ran- 
dom photon flux is given by 



SNR P = Vlp r 



(59) 



where I p is the average photon ar- 
rival rate and t is the time interval 
of the count. When r = l/(2Af) is 
substituted, 



SNR P = Vlp/2Af (60) 

In the case of the photocathode elec- 
tron current, 



SNR D 



fe= /_L 
\2Af \2eAf 



(61) 



where i is the photocathode emission 
current in amperes and e is the 
charge of the electron. If the signal 
current is considered as i, the noise 
in the bandwidth Af for the photo- 
current is the familiar shot noise 
formula {leiAf) 1 / 2 . 

The equation for the photon noise 
squared is given by 



I pN = 2 I p Af 



(62) 



The photocurrent noise squared is 
given by 

IpeN = 2 r, e 2 I p Af (63) 

Both these equations involve the 
photon "current" I p . 

If the photons are not randomly 
emitted, Eq. (62) must be modified. 
In the case in which the variance 
in the photon current is given by 
<r p 2 , the equation becomes 



ipeN 



2e 2 Af 



[.I P (l-,) + ^- 2 ] 



(64) 



Statistical Fluctuation and Noise 



69 



For a randomly emitted photon, the With ij = 0.3 and 5 k = 10 6 , 
noise current squared at the anode 



is given by 

A lis = 2 e 2 r, 5 2k I P Af (65) 

where the multiplier chain is assumed 
to be composed of k high-gain dy- 
nodes exhibiting Poisson statistics, 
each with an average gain 8. 

At the anode, the input resistance 
and capacitance of a preamplifier 
generate a noise current squared 
given by 



I. 



10 i^/(2e 2 q 5 2k Af) (69) 



\l = 4kTg Af 



l + R„g + ^- 2 -| n Af 2 C 2 ] 



(66) 



where g = 1/R is the shunt con- 
ductance in the anode lead, C is the 
shunt capacitance, Af is the band- 
width, k is Boltzmann's constant, T 
is the absolute temperature, and R n 
is the equivalent noise resistance of 
the preamplifier input. The total 
noise current squared through R is 
Al 2 aN + 'n 2 . an d SNR a is given by 

SNR a = rj5 k I p e j\ 2 e 2 7j5 2k I p Af 
+ 4kTgAfM+R„g 



+ 



T»] 



1/2 



(67) 



To maintain the input SNR. Al 2 aN 
must be greater than i n 2 . For ex- 
ample, with Al 2 ax = 10 i 2 „, and 
R = 100 ohms, C*= 100 picofarads, 
R n = 100 ohms, T = 300 °K, and 
Af = 10 6 Hz, at room temperature, 

i£ = 3.2 10-" amperes 2 (68) 



I p ~ 2 X 10 s photons per second 

The value of I p shown in Eq. (69) 
is the lower limit for the average 
photon current and makes the 
squared anode-current fluctuation 
greater than the squared noise cur- 
rent in the photomultiplier preampli- 
fier input by a factor of ten. The 
resulting value of the average photon 
current corresponds to a photoelec- 
tric current of about 10 -14 ampere, 
or about 6 x 10* photoelectrons 
per second. In cases in which the 
dark current is effectively higher 
than 6 x 10 4 photoelectrons per sec- 
ond, the photomultiplier sensitivity 
limit is set by the dark current and 
not by the preamplifier noise. It is 
the noise-free gain of the multiplier 
chain which increases the rms photo- 
current shot noise by a factor of 
8 k and permits the low-input-level 
operation. 



SUMMARY 

The more important expressions 
relating to signal and noise dis- 
cussed in this section are summarized 
below. Beginning at the input to the 
photomultiplier, the signal is followed 
through the tube, and the variance 
and the SNR associated with the 
signal are noted. 

First, a randomly emitted photon 
flux exhibiting Poisson statistics has 
an average value n given by 

n = I p r 

for the number of photons present 
in a time interval t when the aver- 
age rate of emission is I p photons 



70 



RCA Photomultiplier Manual 



per second. The variance or 2 is given 
by 

ffp = I p t = n 

and the signal-to-noise ratio is given 
by 

SNR = Bp/(r p = y/J~r = Vn7 

The number of photoelectrons 
emitted from a photocathode of 
quantum efficiency tj is given by 

Hp.e. = i?I p t 
and the variance is given by 

ff P.e. = »?IpT 

The signal-to-noise ratio of the 
photoelectron flux is given by 



SNR D 



= VvG 



If the photocathode contributes 
randomly emitted thermionic elec- 
trons to the signal of photoelectrons, 
then the variance of the electron 
flux is increased, as follows: 

ff P.e. = rjl v r + H t h, 

where n th is the average number of 
thermionic electrons emitted in a 
time t. The SNR is decreased and 
becomes 

SNRp. e . = 0jI p T)/(r,I p r + n th )» 

The signal-to-noise ratio of the 
multiplier chain with k dynodes 
alone is given by 

SNR k -5L-[f!+.4 + 

Cmk |_°1 ° lS 2 

^ T* 

(Si-$2'"8 k _i) 5y 



where 8j and o-j 2 are the gain and 
variance of the j th stage, respectively, 
and fn k and o- mk are the total gain 
and the total deviation, respectively, 
for the entire multiplier chain. 

For the photomultiplier tube as a 
whole, the anode SNR is given by 

snr & = VflV[i+<ri/ai+«i/(*i-«2) 

((5i-5 2 ---5k-i)5k 2 )]» 

If each stage is identical and has 
Poisson statistics, 

SNR»c^(ijn p («-l)/5)i 

and for large gain (i.e., 8 » 1), 

SNR a ~ Vn%, 

In this case the multiplier chain has 
not reduced the photoelectron SNR. 
If each stage in the multiplier 
chain is identical and has exponen- 
tial statistics, then 

SNR a = hn p (5-l)/25]» 
and for large gain, 

SNR.^ Vi?n p /2 

The SNR for exponential statistics 
is decreased by \/^~2 below that of 
a high-gain multiplier chain with 
Poisson statistics. 

When the photomultiplier is oper- 
ated at high photon counting rates: 

SNR a = J75 k l p e / 2 e 2 »/5 2k Ip Af 
+ 4kTgAfM+R n g 



+ 



4ir 2 



5?APC»Y| 



1/2 



Statistical Fluctuation and Noise 



71 



where I p is the average photon cur- 
rent in photons per second, e is the 
electronic charge, Af is the fre- 
quency bandwidth, T is the absolute 
temperature (°K), k is the Boltz- 
man constant, R n is the equivalent 
noise resistance of the input of the 
preamplifier that processes the anode 
signal, g is the shunt conductance 
in the anode lead, and C is the shunt 
capacitance in the anode lead. 

APPENDIX A- 
STATISTICAL PROCESSES AND 
THE GENERATING FUNCTION 

Statistical processes are character- 
ized by randomly distributed outputs 
for a given input. The relative fre- 
quency of occurrence of any particu- 
lar output is given by the probability 
that this output occurs. As an ex- 
ample, for a single-particle input, 
the output of a particle-multiplication 
device such as a photomultiplier may 
consist of from zero to N particles. 
A particular output n occurs with 
probability P n . The distribution of 
P n sums to unity as follows: 



The expression n/o- is the signal-to- 
noise ratio for the device in ques- 
tion (cr/ii is the relative deviation). 
Obviously, the smaller the value of 
o-, the more precisely determined is 
the output. 

An alternative and more powerful 
method of obtaining the results 
above, as well as others of a more 
complex nature, involves the use of 
the generating function of the dis- 
tribution [P n ]. 6 The generating func- 
tion Q(s) in terms of the auxiliary 
variable s is defined as follows: 



Q(s) = S s» P„ (73) 

n-0 

and has the following properties: 

i a n Q(s) 



N 

SP„ = i 

n— 



(70) 



The average number of output par- 
ticles is given by 



n = 2 n P n 

n-0 



(71) 



On any given single measurement, 
an output consisting of n particles 
is not expected. In fact, n may not 
be integral, even though all possible 
outputs must be. There will be fluc- 
tuations in the output which are 
given by the variance cr 2 , or by the 
rms or standard deviation cr, such 
that 

« = I" 2 (n-5)*.P n l (72) 



Pn = 



n! ds n 



Q(s) |.-i = i 



3Q(s) 



= n 



(74) 



(75) 



(76) 



and 



+ 



6 2 Q(s) I 

_rao(s)| f 
La- U 



9Q(s) 

as 



(77) 



The statistical properties of a 
number of individual devices in 
series (the multiplier chain of a 
photomultiplier is an example) and 
the average output and fluctuation 
for a single device when the input 
consists of more than one particle 
can now be obtained through the 
use of the generating function Q(s) 
of the single-particle-input probabil- 
ity distribution [PJ. 



72 



RCA Photomultiplier Manual 



Fig. 56 shows the series combina- 
tion of two statistical devices. The 
first has an output distribution [P„] A 
which yields an average gain n A and 
variance <t a 2 . The generating func- 
tion for [P n ] A is Qa(s). Similarly, for 
the second device the output distri- 
bution is [P„] B , the average gain is 
n B , the variance is cr B 2 » and the gen- 
erating function is Q B (s). The rela- 
tionships between these individual 
device functions and the total average 
output n AB and the variance cr AB 2 
for the entire serial combination are 
required. The first step is to state 
the generating function Q AB (s) for 
the serial combination shown in Fig. 
56. 



or 

Oab(s) = sp x p; + s 2 (PiPj + p 2 p;pd 
+ s 3 (2p 2 p;pd + s 4 p 2 p;p; 

(79) 

The coefficients of si (j = 1, 2, 3, 4) 
give the probability of a j-particle 
output for the entire series combina- 
tion. Each coefficient comprises the 
sum of all possible ways to obtain 
that particular number of particles 
at the output. 

From Eq. {78), the output prob- 
ability distribution [P„] AB , the total 
average output n AB , and the total 
variance o- AB 2 for the series combina- 
tion can be obtainedfhrough the use 





n AB = 



*AB'«"B>'*A 1+ V'B t 



Fig. 56 — A schematic representation of the serial combination of two statistical 
devices A and B. For a single-particle input the gain and variance of the signal at 
each stage in the device are shown. The total gain and total variance for the 
entire combination in terms of each of the individual device characteristics for a 
single-particle input is given by Eqs. (81) and (84) in the text. 



The generating function for the 
serial combination is given by the 
following relation: 

Qab(s) = Qa[Qb(s)] (78) 

This relation can be understood more 
clearly if the two devices are con- 
sidered to have only two possible 
outputs, either one particle or two 
particles. Then the generating func- 
tions are Q A (s) = sP x + s 2 P 2 and 
Q B (s) = sPi' + s 2 P 2 '. When these 
functions are inserted, Eq. (78) be- 
comes 



Qab(s) = sPip; + s^p; + s»p s p;p; 
+ 2s s p s p;p; + s<p 2 p;p; 



of Eqs. {74), (76), and (77). Of most 
interest are the total average out- 
put and the total variance. From 
Eq. (76), 



nAB 



dQ A 



(80) 



or 



nAB = 



3Qa 3Qb| 
6Qb" da 



[3Qa dOs] 
I 3s ' 6s J 



nA*n B 



(81) 



As expected, the average output for 
the series combination is the product 
of the average outputs of the indi- 



Statistical Fluctuation and Noise 



73 



vidual devices. The variance o- A b 2 
is given by 



Qabcv'z = Qa {Qb [• 



•Qz(s)]| 
(85) 



_2 _ 
ff AB- 



^Qab 3Qab 
ds 2 3s 



3Qab J 
. 3b J 



(82) 



After the differentiation is per- 
formed, 

<t A b = Qa[Qb] 2 + QaQb + QaQb 
- [Qa! 2 [Q'bP (83) 

where the prime indicates differentia- 
tion with respect to s and evaluation 
of the derivative at s = 1. By addi- 
tion and subtraction of a term [Q A ] 
[Q B '] 2 , Eq. (83) can be rearranged 
as follows: 

*ab = [QbNQa + Qa-[Qa] 2 1 

+ Q1[Qb + Qb-[Qb?] 

or, in terms of the individual-device 
average outputs and variances, as 
follows: 

<rl B = [HB] 2 -<ri + nA-4 (84) 

The physical interpretation of Eq. 
(84) is straightforward. If deyice B 
had no fluctuations (o* B 2 — 0). o"ab 2 
would comprise only the variance 
o* A 2 from device A multiplied by 
the square of the average output of 
device B. Each particle from device 
A is multiplied by li B , a constant, 
and therefore the coefficient of o- A 2 
in Eq. (84) is [n B ] 2 . On the other 
hand, if device A had no output 
fluctuations, each of the B A particles 
would undergo independent fluctua- 
tions in device B. As independent 
fluctuations, they add in quadrature, 
and thus o- A b 2 = 5 A o- B 2 - 

More devices could be strung in 
series. In general, for a series of 
devices A, B, C, etc., the generating 
function is given by 



Eqs. (81) and (84) provide a recur- 
sion relation between any two de- 
vices in the series so that the total 
gain and variance from any number 
of devices can be obtained. These 
recursion relations are used in dis- 
cussions of photocathode and mul- 
tiplier-chain statistics. 

When the input to a particular de- 
vice consists of multiple, simultane- 
ous, independent events, the gener- 
ating function for the combination 
is obtained by taking the product of 
the separate generating functions. 
Thus, for two such parallel inputs 
to a statistical device, the correspond- 
ing generating functions are: 



N 

Qa = S s" 



P Am and Qb 



M 

= Ss" 

m— 



Pbh 



When the product is formed, the 
individual coefficients of s^ represent 
the sum of the event probabilities 
which yield j particles in the out- 
put, as follows: 

Qab = Qa-Qb = PaoPbo+(PaoPbi 
+ PaiPbo)s+(PaoPb2 
+ PaiPbi+Pa2Pbo)s ! +"" 

It is obvious that this treatment may 
be extended to multiple simultaneous 
events. In particular, for an m- 
•"particle input, where the generating 
function is Q(s) for each particle, the 
output generating function Q m (s) is 
given by 



Q-(s) = [Q(s)] n 



(86) 



This situation is shown schematically 
in Fig. 57. 



74 



RCA Photomultiplier Manual 



m 


STATISTICAL 

MULTIPLICATION 

DEVICE 


». 


n m = nfi*n 




"m =m-(r 2 



interpret the bulk of the observed 
secondary-emission statistics. 

The distribution has the follow- 
ing form: 



Fig. 57 — A schematic representation 
of a statistical, multiplicative device 
with an m-particle input. The total gain 
and variance for the output in terms 
of the single-particle input character- 
istic are given by Eqs. (87) and (88) 
in the text. 

By use of Eqs. (76) and (77) 

fi m = rn-5 (87) 

and 

<rL = m-o- 2 (88) 

The output probability distribution 
[PJm is obtained by using the m- 
particle generating function [Eq. 
(86)] in Eq. (74). 

Eqs. (87) and (88) can be obtained 
from Eqs. (81) and (84) by assuming 
in that case (see Fig. 56) that the 
first serial device is a perfect multi- 
plier with no variance (o* A 2 = 0). 
Under this restriction, the input to 
device B is a constant m, and the 
gain and variance found from Eqs. 
(81) and (84) are identical to those 
given in (87) and (88). 

APPENDIX B- 

MODEL FOR DYNODE 

STATISTICS 

The observed number distributions 
for secondary electrons vary among 
the different types of dynodes used 
commercially. Nearly all the distri- 
butions fall within the class limited 
by a Poisson distribution 7 at one ex- 
treme and an exponential distribution 
at the other. 4 To describe this wide 
variety of distributions the Polya, or 
compound Poisson, distribution is 
employed. 3 Through the adjustment 
of one parameter, the distribution 
runs from purely Poisson to expo- 
nential. Therefore, this one distribu- 
tion can be used to describe and to 



p(n,b)= gd+bM)-- 1 / 6 n d+jb) 

n. j=1 

(89) 
where P(n, b) is the probability of 
observing n secondaries, /j, is the 
mean value of the distribution, and 
b is the parameter controlling the 
shape of the distribution. With b 
= 0, the distribution is Poisson, as 
follows: 

P(n,o)=£je-* (90) 

With b = 1, an exponential distribu- 
tion results, as given by: 

P(n,l) = „- (l- r -/x)-< n + 1 > (91) 

Fig. 58 shows a family of distribu- 
tions for various values of b. 

The generating function for 
P(n, b) is given by 

Q(s) = [l+bM(l-s)]- 1 ' B (92) 

From Eqs. (76) and (77) of Appen- 
dix A, 

n = M (93) 

and 

a 2 = dm 2 + M (94) 

The fluctuations increase as fi 2 for 
an exponential distribution (b = 1), 
but increase only as /a for the Pois- 
son distribution. The signal-to-noise 
ratio is given by 



= M/(bM 2 +M) J 



(95) 



Fig. 59 shows a log-log plot of the 
signal-to-noise ratio as a function of 
jj, with b as a parameter. The signal- 
to-noise ratio improves with increas- 
ing [i for the Poisson distribution, 
but approaches unity with large fi in 



Statistical Fluctuation and Noise 



75 




20 40 60 80 100 
No. OF OUTPUT PARTICLES, n 



120 



Fig. 58 — Single-particle output dis- 
tribution for a dynode displaying Polya 
statistics. A value of b = 1 gives a 
probably exponential distribution; b — 
gives a Poisson distribution; b = 0.2 
is intermediate between the two extreme 
values. 

the exponential distribution. In fact, 
for any non-zero value of b, the sig- 
nal-to-noise ratio approaches b -1 / 2 
for large fi. As shown in Fig. 59, 
even small departures from Poisson 
statistics significantly reduce the sig- 
nal-to-noise ratio at moderately high 
gains of 10 to 20. It can be antici- 
pated that departures from Poisson 
statistics degrade the single-electron 
pulse-height resolution. 

The Polya distribution has an in- 
teresting interpretation with respect 
to secondary-emission statistics. 3 For 
non-zero values of b, the distributions 



described by Eq. (89) can be shown 
to be composed of a number of dif- 
ferent Poisson processes, each with 
a different mean value. The mean 
values are, in turn, distributed ac- 
cording to the Laplace distribution. 




4 6 8 1 Z 

MEAN GAIN {/.) 



6 8,00 



Fig. 59 — A comparison of the SNR 
as a function of the mean value p for 
a number of Polya distributions. For 
Poisson statistics (b — 0), the SNR in- 
creases as the square root of /«. With 
< b — 1, the SNR approaches the 
square root of b~' for large mean gains. 

When b equals 0, the distribution of 
the mean values collapses to a delta 
function at the value fi, leading to 
a purely Poisson distribution. For b 
equals 1, the distribution of tb,e mean 
values is exponential. The physical 
interpretation for a dynode display- 
ing non-Poisson statistics is that 
physical non-uniformities on the dy- 
node surface cause each element of 
the surface to have a different mean 
value for emission. Although each 
small element exhibits Poisson statis- 
tics with respect to emission, the 
total emission from the entire dy- 
node is non-Poisson because it com- 
prises a distribution of Poisson dis- 
tributions. It is possible that the basic 
emission process from a given dy- 



76 



RCA Photomultiplier Manual 



node is not a Poisson process. How- 
ever, high-gain GaP dynodes exhibit 
nearly Poisson statistics 7 and at 
present it is believed that departures 
from this norm are caused, to a large 
extent, by dynode non-uniformities. 
The departure from Poisson statis- 
tics affects the single-particle pulse- 
height resolution. Fig. 60 shows the 
output-pulse distribution from a sin- 
gle dynode for a number of multiple- 
particle inputs. The resolution is 
clearly degraded in passing from a 
Poisson distribution to an exponen- 
tial one. With the exponential dis- 



SINGLE PARTICLE GAINMOO 
m = INPUT PARTICLE NUMBER 



POISSON DIST.,b = 




6 



200 400 600 800 1000 1200 
No.OF PARTICLES AT OUTPUT (i,) 



Fig. 60 — A comparison of multiple- 
particle distributions from a single dy- 
node having Poisson and exponential 
distribution. Clearly, the particle reso- 
lution characteristics of the exponential 
distribution are much poorer. 



tribution shown in Fig. 60, it would 
be difficult to distinguish among one-, 
three-, and five-particle input pulses, 
whereas the problem virtually dis- 
appears for a Poisson distribution. 

REFERENCES 



1. R. H. Brown and R. Q. Twiss, 
"Interferometry of the Intensity 
Fluctuations in Light", Proc. 
Royal Soc. London, Vol. 242 A, 
p. 300 (1957), Vol. 243A, p. 291 
(1958). 

2. W. Shockley and J. R. Pierce, 
"A Theory of Noise for Electron 
Multipliers", Proc. IRE, Vol. 26, 
p. 321 (1938); V. K. Zworykin, 
G. A. Morton, L. Malter, "The 
Secondary Emission Multiplier — 
A New Electronic Device", Proc. 
IRE, Vol. 24, p. 351 (1936). 

3. J. R. Prescott, Nuc. lnstr. Meth- 
ods, A Statistical Model for photo- 
multiplier Single-Electron Statis- 
tics", Vol. 39, p. 173 (1966). 

4. L. A. Dietz, L. R. Hanrahan and 
A. B. Hance, "Single-Electron Re- 
sponse of a Porous KC1 Trans- 
mission Dynode and Application 
of Polya Statistics to Particle 
Counting in an Electron Multi- 
plier", Rev. Sci. Inst., Vol. 38, 
p. 176 (1967). 

5. M. Schwartz, Information Trans- 
mission, Modulation, and Noise, 
McGraw-Hill, p. 207 (1959). 
T. Jorgensen, "On Probability 
Generating Functions", Am. J. 
Phys., Vol. 16, p. 285 (1948); E. 
Breitenberger, "Scintillation Spec- 
trometer Statistics", Prog. Nuc. 
Phys., Vol. 4, (Ed. O. R. Frisch, 
Pergamon Press, London, p. 56 
(1955). 

G. A. Morton, H. M. Smith, and 
H. R. Krall, "Pulse Height Resolu- 
tion of High Gain First Dynode 
Photomultipliers", Appl. Phys. 
Lett., Vol. 13, p. 356 (1968). 



77 



Application of Photomultipliers 



THIS section discusses the major 
applications of photomultipliers 
and describes some of the special 
considerations for each application. 
The applications covered include 
scintillation counting, Cerenkov radi- 
ation detection, time spectroscopy, 
laser range finding, flying-spot scan- 
ning, detection of low light levels, 
photometry, and spectrometry. This 
listing is not complete even for pres- 
ently known applications, and new 
ones are being continually devised. 
The applications discussed, however, 
are some of the major ones and the 
information given can be readily 
adapted to other applications or to 
new ones. 

SCINTILLATION COUNTING AND 

CERENKOV RADIATION 

DETECTION 

A scintillation counter is a de- 
vice used to detect and register in- 
dividual light flashes caused by ioniz- 
ing radiation, usually in the form of 
an alpha particle, beta particle, 
gamma ray, or neutron, whose en- 
ergy may be in the range from just 
a few thousand electron-volts to 
many million electron-volts. The 
most common use of scintillation 
counters is in gamma-ray detection 
and spectroscopy. 

The gas, liquid, or solid in which 
a scintillation or light flash occurs 



is called the scintillator. A photomul- 
tiplier mounted in contact with the 
scintillator provides the means for 
detecting and measuring the scintil- 
lation. Fig. 61 is a diagram of a 
basic scintillation counter. The three 
most probable ways in which incident 
gamma radiation can cause a scin- 
tillation are by the photoelectric ef- 
fect, Compton scattering, or pair 



.REFLECTOR 
(Al 2 0j POWDER) 




■•-GLASS WINDOW 



PHOTOMULTIPLIER 



Fig. 61 — Diagram of a scintillation 
counter. 

production. The reaction probabili- 
ties associated with each of these 
types of interaction are a function of 
the energy of the incident radiation 
as well as the physical size and atomic 
number of the scintillator material. 
In general, for a given scintillator, 
the photoelectric effect predominates 
at small quantum energies, the 
Compton effect at medium energies, 
and pair production at energies 
above 1.02 MeV. 



78 



RCA Photomuttiplier Manual 



Scintillation Processes 

In the photoelectric effect, a 
gamma-ray photon collides with a 
bound electron in the scintillator and 
imparts virtually all its energy to the 
electron. In the Compton effect, a 
gamma-ray photon with energy E = 
hv interacts with a free electron in 
the scintillator and transmits only 
part of its energy to the electron, as 
shown in Fig. 62. A scattered photon 
of lower energy also results. To sat- 
isfy the conditions of conservation 




Fig. 62 — Compton-effect mechanism. 

of energy and momentum, there is a 
maximum energy that can be trans- 
ferred to the electron. This maximum 
energy, known as the Compton edge, 
occurs when in Fig. 62 is 180 de- 
grees and <f> is degrees; it is given 
by 



Tcm — 



E 



1 + (l/2a) 



(96) 



where E is the photon energy, a = 
E/moC 2 , mo is the rest mass of an 
electron, and c is the speed of light. 
The resultant energy imparted to the 
electron can then range from zero to 
a maximum of T CM . 

In pair production, as shown in 
Fig. 63, the energy of a gamma ray 
is converted to an electron-positron 
pair in the field of a nucleus. The 
gamma ray must have energy at least 
equal to two times the rest-mass- 
energy equivalent of an electron, 2 
(m c 2 ), or 1.02 MeV; any additional 




Fig. 63 — Pair-production mechanism. 

energy is transferred as kinetic en- 
ergy. When the positron is anni- 
hilated, two photons are produced 
180 degrees apart, each with an en- 
ergy of 0.51 MeV. The photons are 
then subject to the normal probabili- 
ties of interaction with the scintil- 
lator. 

In neutron detection, unlike alpha- 
or beta-particle or gamma-ray detec- 
tion, the primary interaction is with 
the nuclei of the scintillator rather 
than its atomic electrons. The inter- 
action may consist of scattering or 
absorption; in either case, some or 
all of the energy of the neutron is 
transferred to the recoil nucleus 
which then behaves similarly to an 
alpha particle. 

In each interaction between a form 
of ionizing radiation and a scintil- 
lator, an electron having some kinetic 
energy is produced. A secondary 
process follows which is independent 
of both the kind of ionizing radiation 
incident on the scintillator and the 
type of interaction which occurred. 
In this secondary process, the kinetic 
energy of the excited electron is dis- 
sipated by exciting other electrons 
from the valence band in the scin- 
tillator material into the conduction 
band. When these excited electrons 
return to the valence band, some of 
them generate light or scintillation 
photons. The number of photons pro- 
duced is essentially proportional to 
the energy of the incident radiation. 
In the photoelectric interaction de- 
scribed above, all of the incident 



Application of Photomultipliers 



79 



photon energy is transferred to the 
excited electrons; therefore, the num- 
ber of photons produced in this sec- 
ondary process, and hence the 
brightness of the scintillation, is pro- 
portional to the energy of the inci- 
dent photon. 



Scintillation Mechanism 

The exact mechanism of the scin- 
tillation or light-producing process is 
not completely understood in all 
types of materials; however, in an 
inorganic scintillator, the phenome- 
non is known to be caused by the 
absorption of energy by a valence 
electron in the crystal lattice and its 
subsequent return to the valence 
band. Fig. 64 shows a simplified en- 
ergy-band diagram of a scintillation 
crystal. The presence of energy levels 
or centers between the valence and 
conduction bands is the result of im- 
perfections or impurities in the crys- 
tal lattice. Three types are important: 
fluorescence centers in which an 
electron, after excitaton, quickly re- 
turns to the valence band with the 
emission of a photon; quenching cen- 



ters in which the excited electron 
returns to the valence band with the 
dissipation of heat without emission 
of light; and phosphorescence centers 
in which the excited electron can be 
trapped in a metastable state until 
it can absorb some additional energy 
and return to the valence band with 
the emission of a photon. An im- 
portant process in the transfer of 
energy to the fluorescence, or activa- 
tor, centers is the generation of ex- 
citons or bound hole-electron pairs. 
These pairs behave much like hydro- 
gen atoms and, being electrically 
neutral, can wander freely through 
the crystal until captured by fluores- 
cence centers. The emission of a 
photon from a phosphorescence cen- 
ter is a relatively slow or delayed 
process. Of the three types of cen- 
ters through which an excited elec- 
tron can return to the valence band, 
the first, fluorescence, is that sought 
for in the preparation of scintillators. 
The second type, quenching, tends to 
lessen the efficiency of the scintilla- 
tor because it does not cause the 
emission of photons, and the third, 
phosphorescence, produces an unde- 
sirable background glow. 



ELECTRONS 



IMPURITY LEVELS — — 
OR TRAPS 



CONDUCTION BAND 




■=f= EXCITED STATES 



I ACTIVATOR CENTER 
1 



GROUND STATE 



.HOLES 
FILLED VALENCE BAND\ 



Fig. 64 — A simplified energy-band diagram of a scintillation crystal: (a) electrons 
in phosphorescence center; (h) electrons in fluorescence center; (c) electrons in 

quenching center. 



80 



RCA Photomultiplier Manual 



Scintillator Materials 

The most popular scintillator ma- 
terials for gamma-ray energy spec- 
trometry is thallium-activated sodium 
iodide, NaI(Tl). This material is 
particularly good because its re- 
sponse spectrum contains a well- 
defined photoelectric peak; i.e., the 
material has a high efficiency or 
probability of photoelectric inter- 
action. In addition, the light emitted 
by the material covers a spectral 
range from approximately 350 to 
500 nanometers with a maximum at 
about 410 nanometers, a range par- 
ticularly well matched to the spectral 
response of conventional photomulti- 
pliers, Nal(Tl) does not, however, 
have a fast decay time in comparison 
to other scintillators, and, therefore, 
is generally not used for fast-time 
resolution. As a comparison, the de- 
cay time constant for Nal(Tl) is 
approximately 250 nanoseconds, 
while for a fast plastic scintillator 
the dominant decay time constant is 
approximately 10 nanoseconds. 

The decay time of a scintillator in- 
volves the time required for all the 
light-emitting luminescence centers to 
return excited electrons to the valence 
band. In some of the better scintilla- 
tors the decay is essentially exponen- 
tial, with one dominant decay time 
constant. Unfortunately, most scin- 
tillators have a number of com- 
ponents each with different decay 
time. 

Collection Considerations 

Because scintillations can occur 
anywhere in the bulk of the scintil- 
lator material and emit photons in 
all directions, there exists the prob- 
lem of collecting as many of these 
photons as possible on the faceplate 
of the photomultiplier. The relative 
standard deviation in the number of 



photons arriving at the faceplate of 
a photomultiplier as a result of a 
single ionizing event E is given by 

relative standard deviation «« 

(NJ'/VN, (97) 

were N p is the average number of 
photons arriving at the faceplate of 
the photomultiplier per incident 
ionizing event. N p is equal to c E f , 
where c is the average number of 
photons per unit energy for the scin- 
tillation, E is the energy of the inci- 
dent radiation, and f is the fraction 
of the total number of photons pro- 
duced which arrive at the faceplate 
of the photomultiplier. 

Eq. (97) implies that it is highly 
desirable that all emitted photons 
be collected at the photomultiplier 
faceplate. This collection problem 
can be simplified by careful selec- 
tion of the shape and dimensions of 
the scintillator to match the photo- 
multiplier photocathode dimensions. 
The coating of all sides of the scin- 
tillator except that which is to be 
exposed to the photomultiplier face- 
plate with a material which is highly 
reflective for the wavelengths of the 
photons emitted by the scintillator 
also proves helpful. Because Nal 
(Tl) is damaged by exposure to air, 
it is usually packaged in an alumi- 
num case lined with highly reflective 
A1 2 3 powder; the NaI(Tl) scin- 
tillator is provided with an exit win- 
dow of glass or quartz. To avoid 
total internal reflection, it is im- 
portant that the indices of refrac- 
tion of the scintillator material, its 
window, any light guide, and the 
photomultiplier faceplate match as 
closely as possible. 

If it is not convenient for the 
photomultiplier to be directly cou- 
pled to the scintillator, as when the 
photomultiplier entrance window is 



Application of Photomuitipiiers 



81 



not flat, light guides can be used. 
Again, care should be taken in de- 
signing the light guide to assure 
maximum light transmission. The 
outer side or surface of the light 
guide should be polished and coated 
with a highly reflective material. In 
some cases, a flexible fiber-optics 
bundle can be used. An optically 
transparent silicone-oil coupling fluid 
should be applied at the scintillator 
(-light guide, if used)-photomultilier 
interface regardless of the light-con- 
duction method used. 

The next important consideration 
in scintillation counting is the con- 
version of the photons to photoelec- 
trons in the photocathode. The 
photocathode should have the great- 
est quantum efficiency possible over 
the spectral range defined by the 
spectral emissivity curve of the scin- 
tillator. The method of determining 
the quantum efficiency of a photo- 
cathode as a function of wavelength 
is explained in the section on Basic 
Performance Characteristics. The bi- 
alkali photocathode K 2 CsSb provides 
the highest quantum efficiency (ap- 
proximately 30 per cent at 385 nano- 
meters) of all photocathodes presently 
available for matching to most scin- 
tillators. 

The uniformity of the photo- 
cathode, i.e., the variation in quan- 
tum efficiency at a given wavelength 
as a function of position on the 
photocathode, is also important. Be- 
cause the number of photoelectrons 
emitted for a constant number of 
photons incident on the photocathode 
is proportional to the quantum effi- 
ciency, any variation in quantum 
efficiency as a function of position 
results in an undesirable variation in 
the number of photoelectrons emitted 
as a function of position. 

When the scintillation is fairly 
bright, i.e., when a large number of 
photons are produced per scintilla- 



tion, and when the scintillator is 
thick in comparison to its diameter, 
as shown in Fig. 65(a), the photo- 
cathode is approximately uniformly 
illuminated during each scintillation. 
However, if the scintillator is thin 
in comparison to its diameter, as 
shown in Fig. 65(b), or if the scin- 
tillation is very weak, the illumination 
of the photocathode as a function 
of position is closely related to the 
position of origin of the scintillation; 
therefore, photocathode uniformity is 
much more important when a thin 
scintillator is used/ Photocathode 
uniformity also becomes more im- 
portant as the energy of the inci- 
dent radiation becomes less and the 
number of photons per disintegra- 
tion is reduced. 

In some photomuitipiiers, the col- 
lection efficiency for photoelectrons 
decreases near the outer edges of the 
photocathode; therefore, best results 
are obtained when the scintillator is 
slightly smaller in diameter than the 
photocathode. 




COUPLING 
FLUID 



PHOTOCATHODE 



REFLECTIVE 
COATING 



(a) 




COUPLING 
FLUID 



PHOTOCATHODE 



(*) 



Fig. 65 — Scintillator geometries: (a) 

thick in comparison to diameter; (b) 

thin in comparison to diameter. 



82 



RCA Photomultiplier Manual 



Multiplier Considerations 

The two multiplier configurations 
most commonly used in photomulti- 
pliers in scintillation-counter appli- 
cations are the venetian-blind and 
focused (either in-line or circular- 
cage) structures. The advantage of 
the venetian-blind structure is that 
the useful portion of the first dynode 
area is quite large in comparison to 
the photocathode area. On the other 
hand, the focused structure has a 
small first-dynode area and, there- 
fore, requires a more complex front- 
end design to provide good collection 
efficiency. However, a well designed 
focused structure, although more 
complex, can provide a better collec- 
tion efficiency than the venetian- 
blind structure. In addition, the fo- 
cused structure is several times 
faster than the venetian-blind type, as 
discussed in the section on Principles 
of Photomultiplier Design. 



Significant Photomultiplier 
Characteristics 

Photomultiplier dark noise is of 
particular importance in scintillation- 
counting applications when the en- 
ergy of the ionizing radiation is small, 
or when very little energy is trans- 
ferred to the scintillation medium; in 
short, when the flash per event rep- 
resents only a few photons. If a 
photomultiplier is coupled to a scin- 
tillator and voltage is applied, the 
composite of all noise pulses coming 
from the photomultiplier is referred 
to as the background of the system. 
The plot of a frequency distribution 
of these pulses as a function of en- 
ergy is shown in Fig. 66. The well- 
defined peaks seen in the figure are 
caused by external radiations and 
can be reduced by placing the photo- 
multiplier and scintillator in a lead 
or steel vault to reduce background 
radiation. 



UJ 

z 
z 
< 



1" 
M 

x o 
o 
4 a 

ffi u 

m 



662 KeV 

PULSE HEIGHT 



1.46 MeV 1.76 MeV 



Fig. 66 — Distribution of radiation background pulses as a function of en- 
ergy. A calibration spectrum showing the Cs 1 " photopeak is included for 

reference. 



Application of Photomultipliers 

It is desirable that the background 
of the scintillation counter be as low 
as possible. Photomultipliers have 
been developed in which the back- 
ground is kept low by the use of 
radioactively clean metal cans in- 



13 

liquid scintillation counting has de- 
veloped rapidly. The beta-ray ener- 
gies from these radioisotopes may 
vary over a fairly broad range; for 
example, H 3 emits betas with ener- 
gies varying from zero to. a maximum 



surnimmn. 



lire mice mosi com- "3 »«= iwiuwmg c 4 uauuii: 



monly used radioisotopes in liquid 
scintillation counting are tritium, H 3 ; 
carbon, C 14 ; and phosphorus, P 32 . 
Because these elements are the most 
suitable radioactive tracers used in 
biological experiments, the field of 

* Registered Trade Name for General 
Electric Co. material. 



C = 



2N!N 2T 
60 



(98) 



where C is the chance coincidence 
rate per minute, N x is the dark-noise 
count rate in counts per minute from 
tube No. 1, N 2 is the dark-noise count 



84 



RCA Photomultiplier Manual 




ENERGY (COMPRESSED SCALE) 
Fig. 67 — Scintillation count showing distribution of beta-ray energies. 



SCINTILLATOR 



PHOTOMULTiPLIER 



PHOTOMULTIPLIER 



T~ IT 



AMPLIFIER 



PULSE ADDER I- 



■j DISCRIMINATOR] 



COINCIDENCE 
GATE 



Fig. 68 — Block diagram of a liquid-scintillation spectrometer. 



Application of Photomultipliers 



85 



rate in counts per minute from tube 
No. 2, and t is the resolving time 
of the coincidence circuit in seconds. 
In a liquid-scintillation spec- 
trometer employing two tubes with 
dark-noise ratings of 30,000 counts 
per minute each and a coincidence 
circuit having a resolving time of 10 
nanoseconds, the number of acci- 
dental coincidences is approximately 
0.3 count per minute. Scintillations 
resulting in at least one photoelectron 
being emitted from the photocathode 
of each tube should produce a sig- 
nal pulse from the coincidence cir- 
cuit. Because a scintillation of at 
least two photons is required to ini- 
tiate a pulse from both photomulti- 
pliers, the over-all detection efficiency 
for both tubes together is slightly 



lower than for a single tube having 
a very low background count rate. 

When a vial filled only with a scin- 
tillator is placed between the photo- 
multipliers, and the output from the 
coincidence circuit is examined by 
use of a multichannel analyzer, a 
pulse-height distribution such as that 
shown in Fig. 69 is obtained. Clearly, 
not many of the background pulses 
shown are caused by the accidental 
coincidences of dark-noise pulses 
from the photomultipliers, but are 
caused by cosmic rays or scintilla- 
tions in the material of the vial and 
photomultiplier envelope resulting 
from the presence of radioisotopes in 
the materials of which they are con- 
structed. Light originated internally 
in either photomultiplier and trans- 




PULSE HEIGHT 
(COMPRESSED SCALE) 

Fi g- 69 — Background distribution obtained from a liquid scintillator using 
a coincidence system. 



RCA Photomultiplier Manual 



mitted to the other also causes back- 
ground pulses; this phenomenon is 
sometimes referred to as crosstalk. 

The counting efficiency E for a 
liquid scintillation counter is de- 
fined in per cent as follows: 

(counting rate from coinci- 
E _ dence circuit)-(background) 
disintegration rate of sample 

(99) 

The best counting efficiency for 
a given radioisotope is obtained when 
a highly efficient scintillator and a 
photomultiplier with a high quantum 
efficiency in the spectral emissivity 
range of the scintillator are used. 
The optical system containing the 
two photomultipliers and the count- 
ing vial is also of major importance; 
it should be designed so that, as far 
as possible, the photons produced in 
a scintillation are equally divided be- 
tween the photomultipliers. This di- 
vision assures that a coincidence 
pulse results from as many scintilla- 
tions as possible. In some cases, two 
or more different isotopes may be 
counted simultaneously; therefore, it 
is desirable that the photomultipliers 
have matched gains and good energy 
(pulse-height) resolution capability to 
provide best isotope separation. 

A figure of merit EVb has been 
developed to aid in the evaluation of 
systems used in liquid scintillation 
counting; E is the counting effi- 
ciency of the system for a given iso- 
tope, and b is the background of 
the system in an energy range in- 
cluding the energy range of pulses 
from the radioisotope. The figure of 
merit is, however, not necessarily the 
best way of evaluating all systems. 
It is valid at very low counting rates 
where the background is the domi- 
nant factor, but is not of great help 
at high counting rates where the 
background of the system becomes 



less important than the efficiency in 
determining the merit of the system. 
In selecting a photomultiplier tube 
for a liquid-scintillation application, 
the following items are of major im- 
portance: high photocathode quan- 
tum efficiency, low dark-noise count 
rate, minimum internal-light genera- 
tion, low radioactive-background en- 
velope, low scintillation-efficiency 
envelope, fast time response, and 
good energy resolution. The RCA- 
450 1V3 photomultiplier has been 
specifically designed to meet these 
requifements. 

Cerenkov Radiation Detection 

Cerenkov radiation is generated 
when a charged particle passes 
through a dielectric with a velocity 
greater than the velocity of light in 
the dielectric (i.e., v greater than 
c/n, where n is the index of refrac- 
tion of the dielectric). Polarization of 
the dielectric by the particle results 
in the development of an electromag- 
netic wave as the dielectric relaxes. 
If the velocity of the particle is 
greater than the velocity of light, 
constructive interference occurs and 
a conical wavefront develops, as il- 
lustrated in Fig. 70. Cerenkov radia- 
tion is the electromagnetic counter- 
part of the shock wave produced in 
a gas by an object traveling faster 
than sound. It is highly directional, 
and occurs mostly in the near-ultra- 
violet part of the electromagnetic 
spectrum. Because the radiation is 
propagated in the forward direction 
of motion of the charged particle, as 
shown in Fig. 70, Cerenkov detec- 
tors can be made to detect only those 
particles that enter the system from 
a restricted solid angle. 

Cerenkov radiation produced in 
an aqueous solution by beta emitters 
can be useful in radioassay tech- 
niques because it is unaffected by 



Application of Photomultipliers 



87 




DIRECTION OF 
MOTION OF 
CHARGED PARTICLE 



Fig. 70 — Diagram of the Cerenkov 
radiation mechanism. 

chemical quenching and because it 
offers the advantages, over liquid- 
scintillation counting techniques, of 
simplified sample preparation and 
the ability to accommodate large- 
volume samples. Because a fast par- 
ticle is required to produce Cerenkov 
radiation, rather high-energy beta 
rays are required; e.g., the threshold 
for Cerenkov radiation is 261 keV 
for electrons in water. Because the 
photon yield for Cerenkov light is 
usually very low, the same considera- 
tions concerning photomultiplier se- 
lection apply for the Cerenkov de- 
tection as for liquid-scintillation 
counting. The tube selected should 
also be equipped with a faceplate 
capable of good ultraviolet trans- 
mission. 

Time Spectroscopy 

In addition to the energy spectros- 
copy described above, there are oc- 
casions when it is of advantage to 



measure time differences such as be- 
tween a pair of gamma rays or a 
combination of gamma rays and par- 
ticles in cascade de-exciting some 
level in a nucleus. In time spectros- 
copy, some special considerations 
must be made in selecting the scin- 
tillator, photomultipliers, and tech- 
nique of analyzing the signals from 
the photomultipliers. 

In a photomultiplier, time resolu- 
tion is proportional to (n) _1/2 , where 
n is the average number of photo- 
electrons per event. It is therefore 
important to choose a scintillator 
material that provides a high light 
yield for a given energy of detected 
radiation. It is also important that 
the variation of the time of interac- 
tion of the radiation with the scintil- 
lator be as small as possible. This 
minimum variation is assured by at- 
tention to scintillator thickness and 
source-to-detector geometry. The de- 
cay time constant of the light-emit- 
ting states in the scintillator should 
be as short as possible, and the 
geometry and reflective coatings of 
the scintillator should be selected so 
that variations in path lengths of 
photons from the scintillator to the 
photocathode of the photomultiplier 
are minimized. The photocathode of 
the photomultiplier selected should 
have a high quantum efficiency. In 
addition, the transit-time dispersion 
or jitter (variations in the time re- 
quired for electrons leaving the 
photocathode to arrive at the anode 
of the tube) should be small over 
the entire photocathode area. 

The major contribution to transit- 
time spread occurs in the photo- 
cathode-to-first-dynode region and 
may be a result of the initial kinetic 
energies of the emitted photoelec- 
trons and their angle of emission. 
Focusing aberrations, and the single- 
electron response or rise time, i.e., 
the output-pulse shape at the anode 



88 



RCA Photomultiplier Manual 



for a single photoelectron impinging 
on the first dynode, may also be of 
some importance. Although the sin- 
gle-electron response theoretically 
does not have much effect on time 
resolution, it does change the trigger- 
ing threshold at which the best time 
resolution can be obtained. 

The most commonly used time- 
spectroscopy techniques include lead- 
ing-edge timing, zero-crossover tim- 
ing, ■ and constant-fraction-of-pulse- 
height-trigger timing; the technique 
used depends on the time resolution 
and counting efficiency required and 
the range of the pulse heights en- 
countered. A block diagram of a 
basic time spectrometer is shown in 
Fig. 71. 

Leading-edge timing makes use of 
a fixed threshold on the anode-cur- 
rent pulse and provides good time 
resolution over a narrow range of 
pulse heights. The fractional pulse 
height F at which the triggering 
threshold is set is defined as follows: 



F = 



V t 



(100) 



where V t is the discriminator thresh- 
old, and V a is the peak amplitude of 
the anode-current pulse. The frac- 
tional pulse height has a consider- 
able effect on the time resolution ob- 
tained; best results are usually 
obtained with F equal to 0.2. 

In fast zero-crossover timing, the 
anode pulse is differentiated twice. 
This double-differentiation produces 
a bipolar output pulse that triggers 
the timing discriminator at the zero 
crossover, the time required to col- 
lect approximately 50 per cent of 
the total change in the photomulti- 
plier pulse. Zero-crossover timing is 
second to leading-edge timing for 
time-resolution work with narrow 
pulse-height ranges, but is better 
than the leading-edge method for 
large pulse-height ranges. 

In constant-fraction triggering, the 
point on the leading edge of the 
anode-current pulse at which lead- 
ing-edge timing data indicate that the 
best time resolution can be obtained 
is used regardless of the pulse height. 
For this reason, constant-fraction-of- 
pulse-height timing is the best method 



TIME 

PICKOFF 

DEVICE 



SOURCE OF 
COINCIDENT 
RADIATION 



r~ 



TIME 

PICKOFF 

DEVICE 



TIMING 
PULSE 

-ur 



NUMBER 
OF EVENTS 



START t OUTPUT PULSE 

HEIGHT IS PROPOR- 
TIONAL TO TIME. 

-Or 



TIME TO 
PULSE HEIGHT 
CONVERTER 



u STOP 



TIMING 
PULSE I 

"LfoELAI 



il 



MULTICHANNEL 
(PULSE HEIGHT) 
ANALYZER 



TIME 
SPECTRUM 



Fig. 71 — Generalized block diagram of a time spectrometer. 



Application of Photomultipliers 



89 



for obtaining optimum time resolu- 
tion no matter what the pulse-height 
range. 

Interpretation of 
Counting Data 

A number of techniques can be 
used to interpret the data obtained 
from a scintillation counter; one of 
the most popular employs a multi- 
channel analyzer. The analyzer sorts 
the pulses according to their ampli- 
tude and records the number of 
pulses in a "channel" whose position 
is proportional to the amplitude of 
the pulse. Many pulses are analyzed 
in this manner and a frequency dis- 
tribution of pulse heights is obtained. 
Fig. 72 shows a pulse-height distri- 
bution for a scintillation counter 
counting pulses from a sample of 
Cs 13T . 




a. 32 keV Barium x-ray line. 

b. Backscatter peak. 

c. Compton edge. 

d. Photopeak, cesium 137, 0.662 MeV. 

e. 749 keV photopeak of cesium 134 
impurity. 

/. Continuum due to accidental self- 
coincidences producing partial sum 
pulses. 

g. Accident sum coincidence peak. 

h. Detail due to background radiation. 

Fig. 72 — Pulse-height distribution for 

a scintillation counter counting pulses 

from a sample of Cs"°. 



LASER RANGE FINDING 

A simplified block diagram of a 
laser range finder is shown in Fig. 
73. The interference filter is used to 
pass the wavelength of radiant en- 
ergy of the laser with a minimum of 
background radiation. Photomulti- 
pliers used in laser range finding may 
have a relatively small photocathode, 
but they must exhibit high quantum 
efficiency, low dark noise, and fast 
rise time or an equivalent large 
bandwidth. Most photomultipliers 
can provide bandwidths exceeding 
100 MHz and at the same time main- 
tain relatively large output signals. 
The bandwidth of a photomultiplier 
can be limited by the RC time con- 
stant of the anode circuit. 

The range of a laser-range-finding 
system depends on system parameters 
and operational environment. The 
maximum range of a given system 
may be signal-photon limited or 
background limited. The photon- 
limited case exists when the back- 
ground and detector noise can be 
considered negligible. The maximum 
range in this case is determined by 
the signal-to-noise ratio in the photo- 
electron pulse corresponding to the 
scattered laser return beam. As noted 
in the section on Statistical Fluctua- 
tion and Noise, the signal-to-noise 
ratio of such a pulse is proportional 
to the square root of the product of 
the number of incident photons on 
the photomultiplier and the quan- 
tum efficiency of the photoelectric 
conversion. If the atmospheric at- 
tenuation is neglected and it is as- 
sumed that the laser spot falls 
entirely within the target, the maxi- 
mum range in this case would be in- 
creased as the square root of the 
quantum efficiency of the photo- 
cathode. This increase follows be- 
cause the number of photons col- 
lected by the aperture of the receiving 



90 



RCA Photomultiplier Manual 



TRIGGER 
GENERATOR 



RANGE, TIME 

AND COUNTING 

CIRCUITS WITH 

INDICATOR 




Fig. 73 — Simplified block diagram of a laser range finder. 



system varies inversely as the square 
of the distance from the target. Re- 
cent improvements in the multialkali 
photocathode, resulting in the 
ERMA cathode, described in the 
section on Photoemission and Sec- 
ondary Emission, have increased the 
quantum efficiency at 860 nano- 
meters from 0.01 per cent to 1.5 per 
cent. In terms of laser range in the 
photon-limited case, this quantum- 
efficiency improvement implies more 
than a 10-to-l range increase. The 
use of a GaAs photocathode is even 
more promising at this wavelength. 
In cases where the laser pulse must 
be detected against the background 
of a daylight scene, it is said to be 
background limited. If atmospheric 
attenuation again is neglected, the 
signal is proportional to the number 
of incident photons times the quan- 
tum efficiency of the photocathode. 
The number of incident photons 
from the return beam is inversely 
proportional to the square of the 
range, again assuming that the tar- 
get is larger than the laser spot. The 
noise, however, is independent of 
the range and is determined by the 
square root of the product of the in- 
cident background radiation and the 
quantum efficiency. Thus, to a first 



approximation the range is increased 
in the background-limited case only 
by the fourth root of the quantum 
efficiency. Because the photomulti- 
plier current caused by the back- 
ground radiation is proportional to 
the solid angle of the scene from 
which the photomultiplier collects 
radiation, the background current in 
the photomultiplier may be mini- 
mized by the use of a photocathode 
having a small area or by the use of 
a limiting aperture on the faceplate 
of the photomultiplier. No loss in 
collected laser light need result be- 
cause the return beam may generally 
be considered as originating from a 
point source. The system aperture, 
of course, must be large enough to 
avoid optical alignment problems. 

FLYING-SPOT SCANNING 

An important application for 
which photomultipliers are particu- 
larly well suited is flying-spot scan- 
ning, a system for generating video 
signals for television display from a 
photographic transparency. The ele- 
ments of a flying-spot scanning sys- 
tem are shown in Fig. 74. A cathode- 
ray tube, in conjunction with its 
power supplies and deflection cir- 



Application of Photomultipliers 



91 



cuits, provides a small rapidly mov- 
ing light source which forms a raster 
on its face. This raster is focused by 
the objective lens in the optical sys- 
tem onto the object being scanned, a 
slide transparency or a motion- 
picture film. The amount of light 
passing through the film varies with 
the film density. This modulated light 
signal is focused upon the photomu- 
tiplier by means of the condensing- 
lens system; the photomultiplier con- 
verts the radiant-energy signal into 
an electrical video signal. The ampli- 
fier and its associated equalization 
circuits increase the amplitude of the 
video signal as required. 

The flying-spot scanning system is 
capable of providing high-resolution 
monochromatic performance.- With 
the addition of (a) appropriate di- 
chroic mirrors which selectively re- 
flect and transmit the red, blue, and 
green wavelengths, (b) two addi- 
tional photomultipliers with video 
amplifiers, and (c) appropriate fil- 
ters, color operation is possible. In 
color operation, the primary wave- 
lengths are filtered after separation 



by the light-absorbing filters before 
being focused upon each of three 
photomultipliers, one for each color 
channel. The output of each photo- 
multiplier is then fed to a separate 
video amplifier. 

Flying-spot video-signal genera- 
tors are used in the television indus- 
try primarily for viewing slides, test 
patterns, motion-picture film, and 
other fixed images. Systems have 
been developed for the home-enter- 
tainment industry that allow slides 
and motion-picture film to be shown 
on the picture tube of any type of 
commercial television receiver. 

Cathode-Ray Tube 

Several important considerations 
must be taken into account if the 
cathode-ray tube in the system is 
to produce a light spot capable of 
providing good resolution. The cath- 
ode-ray tube should be operated with 
as small a light spot as possible. The 
cathode-ray-tube faceplate should be 
as blemish-free as possible and the 
tube should employ a fine-grain phos- 



VERTICAL a HORIZONTAL 
SAWTOOTH GENERATORS 




TO LINE 
AMPLIFIER 



Fig. 74 — Elements of a flying-spot scanning system. 



92 



RCA Photomultiplier Manual 



phor; blemishes adversely affect sig- 
nal-to-noise performance and con- 
tribute to a loss of resolution. 

The spectral output of the cath- 
ode-ray-tube phosphor should match 
the spectral characteristic of the 
photomultiplier. This match can be* 
rather loose in a monochromatic fly- 
ing-spot generator; however, the 
spectral output of the phosphor of 
the cathode-ray tube used in a three- 
color version must include most of 
the visible spectrum. Phosphors 
used in monochromatic systems may 
provide outputs in the ultraviolet re- 
gion of the spectrum and still per- 
form satisfactorily. Phosphors such 
as P16 and P15, when used with an 
appropriate ultraviolet filter, display 
the necessary short persistence re- 
quired in a monochromatic system. 

The visible portion of the PI 5 
or P24 phosphor is used in color 
systems. The PI 5 and PI 4 phos- 
phors are, however, much slower 
than the PI 6 phosphor and cause a 
lag in buildup and decay of output 
from the screen. 

The phosphor lag results in trail- 
ing, a condition in which the per- 
sistence of energy output from the 
cathode-ray tube results in a con- 
tinued and spurious input to the 
photomultiplier as the flying spot 
moves across the picture being 
scanned. The result is that a light 
area may trail into the dark area in 
the reproduced picture. 

Similarly, the lag in buildup of 
screen output causes a dark area to 
trail over into the light area. The re- 
sult of these effects on the repro- 
duced picture is an appearance 
similar to that produced by a video 
signal deficient in high frequencies; 
consequently, high-frequency equali- 
zation is necessary in the video 
amplifier. 



Objective Lens 

The objective lens used in a fly- 
ing-spot generator should be of a 
high-quality enlarger type designed 
for low magnification and, depend- 
ing upon the cathode-ray-tube light 
output, should be corrected for ultra- 
violet radiation. The diameter of the 
objective lens should be adequate to 
cover the slide to be scanned. An 
enlarging f/4.5 lens with a focal 
length of 100 millimeters is suit- 
able for use with 35-millimeter slides. 



Photomultiplier Tube 

The spectral characteristics of the 
photomultiplier (or photomultipliers 
in the case of the three-color sys- 
tem) and the cathode-ray-tube phos- 
phor should match. Usually an S-4 
or S-ll spectral response is suitable 
for use in a monochromatic system 
or as the detector for the blue and 
green channels; an S-20 response is 
very often utilized for the red chan- 
nel. The bialkali cathode (K-Cs-Sb) 
is also well suited for the blue chan- 
nel. The speed of the detector must 
be sufficient to provide the desired 
video bandwidth. Most requirements 
do not exceed 6 to 8 MHz, a figure 
well within photomultiplier capabili- 
ties. 

The anode dark current of the 
photomultiplier should be small com- 
pared to the useful signal current. 
The signal-to-noise ratio will be 
maximized by operation of the photo- 
multiplier at the highest light levels 
possible. If necessary, the over-all 
photomultiplier gain should be re- 
duced to prevent excessive anode 
current and fatigue. 

The amplitude of the light input 
is usually a compromise between an 
optimum signal-to-noise ratio and 
maximum cathode-ray-tube life, 
which may be reduced as a result 



Application of Photomultipliers 



93 



of loss of phosphor efficiency at high 
beam-current levels. The signal-to- 
noise ratio can also be improved by 
the selection of photomultipliers hav- 
ing the highest photocathode sensi- 
tivities possible. However, because 
the spread of photocathode sensitivi- 
ties is seldom greater than two or 
three to one, the improvement af- 
forded by such selection is limited 
to two or three dB, an improvement 
difficult to detect while observing a 
television display but desirable and 
necessary in some critical applica- 
tions. 

Video Amplifier 

Photomultiplier gain need only be 
sufficient to provide a signal of the 
required level to the succeeding 
video-amplifier stages. These stages, 
in addition to providing the necessary 
amplification and bandwidth to as- 
sure good picture quality, incorporate 
equalization circuits composed of 
networks with different time con- 
stants. The relatively long decay 
time of these circuits generally re- 
sults in appreciable reduction of the 
useful signal-to-noise ratio. There- 
fore, the use of short-persistence 
phosphors is recommended to re- 
duce the required amount of equali- 
zation. 

In addition to the video amplifier, 
a gamma-correction amplifier is re- 
quired in each channel. The gamma- 
correction amplifier assures maxi- 
mum color fidelity by making the 
linearity or gamma of the system 
unity. 

LOW-LIGHT-LEVEL DETECTION 

Systems for the detection of low 
light levels make use of two basic 
techniques: charge integration, in 
which the output photocurrent is 
considered as an integration of the 



anode pulses which originate from 
the individual photoelectrons, and 
the digital technique in which indi- 
vidual pulses are counted. 

Charge-Integration Method 

In the charge-integration method, 
either the transit-time spread of the 
photomultiplier or the time charac- 
teristics of the anode circuit cause 
the anode pulses to overlap and pro- 
duce a continuous, though perhaps 
noisy, anode current. The current is 
modulated by turning the light off 
and on by means of some mechanical 
device such as a shutter or light 
"chopper". The signal becomes the 
difference between the current in the 
light-on and light-off conditions. 

Detection is limited by noise in 
the anode current. At low light levels 
the noise is caused primarily by the 
fluctuations in dark current of the 
photomultiplier (as discussed in the 
sections on Basic Performance Char- 
acteristics and Statistical Fluctuation 
and Noise). The noise may be mini- 
mized by reducing the bandwidth of 
the measuring system. For example, 
a dc system may be used with a 
bandwidth of only a few hertz if an 
appropriate low-level current meter 
is selected. Bandwidth can also be 
reduced by some technique of av- 
eraging the current fluctuations over 
a period of time. 

Another technique of charge inte- 
gration is to chop the light signal 
with a motor-driven chopper disk 
having uniformly spaced holes or 
slots. The output current is then fed 
through an amplifier having a narrow 
bandwidth tuned to the frequency of 
chop. Bandwidths of the order of 1 
Hz are typical. 

The earlier discussions on Equiva- 
lent Noise Input and on Noise 
Formulas Relative to Photomulti- 
pliers contain the information neces- 



RCA Photomultiplier Manual 



sary to evaluate the performance of 
photomultipliers in the charge- 
integration method. 

Digital Method 

In the digital method for the de- 
tection of low light levels a series 
of output pulses, each corresponding 
to a photoelectron leaving the photo- 
cathode of a photomultiplier, appears 
at the anode. All of the output pulses 
from the tube are- shaped by a pre- 
amplifier before they enter a pulse- 
amplitude discrimination circuit. 
Only those pulses having amplitudes 
greater than some predetermined 
value and having the proper rise-time 
characteristics pass through to the 
signal-processing circuits. The digital 
technique is superior to charge inte- 
gration at very low light levels be- 
cause it eliminates the dc leakage 
component of the dark current as 
well as dark-current components 
originating at places other than the 
photocathode. Fig. 75 shows a digi- 
tal system in block form. 

In the special case in which the 
digital technique is used to count 
single photons incident upon the 
photocathode of a photomultiplier, 
signal pulses appear at the anode 
with an average pulse amplitude PH 
equal to em, where e is the electron 
charge and m is the photomultiplier 
gain. The number of signal pulses 
N a arriving at the anode is given by 



where N p is the number of photons 
incident on the photocathode win- 
dow, and tjX is the quantum effi- 
ciency of the photocathode at the 
photon wavelength including a fac- 
tor for the loss of light by reflection 
and absorption, and a factor for the 
loss resulting from imperfect elec- 
tron-collection efficiency of the front 
end of the photomultiplier. 

The dark-noise pulses present in 
addition to the signal pulses originate 
mainly from single electrons and 
have a pulse-height distribution as 
shown in the simplified dark-noise 
pulse-height-distribution spectrum of 
Fig. 76. Region A of Fig. 76 in- 
cludes circuit-originated noise, some 




LOWER 

DISCRIMINATOR 

LEVEL 



I ELECTRON 

|_UPPER 



DISCRIMINATOR 
LEVEL 



PULSE HEIGHT 



N. 



NpTjX 



Fig. 76 — Dark-noise pulse-height-dis- 
tribution spectrum. 

single-electron pulses, and pulses 
caused by electrons originating at the 
(101) dynodes in the multiplier section. 







PREAMPLIFIER 

PULSE AND 

SHAPER 


PHOTOMULTIPLIER 











ADJUSTABLE 

PULSE AMPLITUDE 

DISCRIMINATOR 



TO 
-PROCESSING 
CIRCUITS 



Fig. 75 — Block diagram illustrating a digital system for detection of low light 

levels. 



Application of Photomultipliers 

Pulses originating at the dynodes 
exhibit less gain than the single-elec- 
tron pulses from the cathode. Region 
B represents the single-electron 
pulse-height distribution and is the 
region in which the single-photon 
signal pulses appear. Region C is 
caused by cosmic-ray muons, after- 
pulsing, and radioactive contami- 
nants in the tube materials and in the 
vicinity of the tube. To maximize 
the ratio of signal pulses to noise 
pulses in single-photon counting, 
lower- and upper-level discriminators 
should be located as shown in the 
figure. 

If the rate of photon arrival is I p) 
the net quantum efficiency is 17, and 
the counting interval is t, the num- 
ber of signal pulses is I p tjt. The rate 
of single-electron dark pulses is re- 
ferred to as I d . At the limit of detec- 
tion the number of dark pulses will 
greatly exceed the pulses which are 
photon originated. Two counts must 
be made, one with the light on and 
one with it off. The difference in 
the number of counts is the signal, 
I p t/t, in numbers of electrons for 
the count interval. The variance for 
this measurement is given by 



ff 2 = I p ,, T + I d r + la r (102) 

The reason for the double entry of 
the variance for the dark counts I d T 
is that the determination involves the 
subtraction of the light-on and light- 
off counts. If it is assumed that the 
number of dark pulses greatly ex- 
ceeds the light-originated pulses, the 
signal-to-noise ratio is given by the 
following expression: 



SNR = 



[2Idl» 



(103) 



95 

(The section on Statistical Fluctua- 
tion and Noise contains a further 
discussion of the statistics of photon 
counting.) 

The signal-to-noise ratio may be 
improved by the square root of the 
time of count. Increasing the time 
of count is analogous to decreasing 
the bandwidth in the charge-integra- 
tion method. The signal-to-noise 
ratio can also be improved by a 
square-root factor by decreasing the 
number of dark-noise pulses. Be- 
cause these pulses originate at the 
photocathode surface, the number 
can be reduced by reducing the area 
of the photocathode or by reducing 
the effective photocathode area by 
using electron optics to image only 
a small part of the photocathode on 
the first dynode. It may also be de- 
sirable to cool the photomultiplier 
and thereby reduce the thermionic 
emission from the photocathode (as 
discussed in the section on Basic 
Performance Characteristics). 

Very-Low-Light-Level 
Photon-Counting Technique 

Before beginning very-low-light- 
level photon counting, the following 
special precautions must be taken: 

1. The power supply and intercon- 
necting circuits must have low-noise 
characteristics. 

2. The optical system must be 
carefully designed to minimize pho- 
ton loss and to prevent movement 
of the image of the object on the 
photocathode, a possible cause of 
error if the cathode is non-uniform. 
It is generally good practice to de- 
focus the image on the photocathode, 
especially in the case of a point 
source, to minimize problems which 
may result from a non-uniform 
photocathode. 

3. The photomultiplier must be 
allowed to stabilize before photon 



96 



RCA Photomultiplier Manual 



counting is begun. The tube should 
not be exposed to ultraviolet radia- 
tion before use and should, if pos- 
sible, be operated for 24 hours at 
the desired voltage before the data 
are taken. 

4. The photomultiplier should be 
operated with the cathode at ground 
potential if possible. If the tube is 
operated with the photocathode at 
negative high voltage, care must be 
taken to prevent the glass envelope 
of the tube from coming into con- 
tact with conductors at ground po- 
tential or noisy insulators such as 
bakelite or felt. Without this pre- 
caution, a very high dark noise may 
result as well as permanent damage 
to the photocathode. 

5. Large thermal gradients must 
not be permitted across the tube as 
it is cooling. In addition, care must 
be taken to avoid excessive condensa- 
tion across the leads of the tube or 
on the faceplate. 

Photomultiplier Selection 
for Photon Counting 

In the selection of a photomulti- 
plier for use in photon counting, sev- 
eral important parameters must be 
considered. First, and most impor- 
tant, the quantum efficiency of the 
photocathode should be as high as 
possible at the desired wavelength. 
To minimize the thermionic dark- 
noise emission, the photocathode area 
should be no larger than necessary 
for signal collection; the multiplier 
structure should utilize as large a 
fraction as possible of the electrons 
from the photocathode. The over-all 
tube should have as low a dark noise 
as possible. In some applications, the 
rise time and time-resolution capa- 
bilities of the tube may also be im- 
portant. 

A number of more recent develop- 
ments in photomultiplier design are 



of considerable significance in pho- 
ton counting. These developments 
fall into two major categories, sec- 
ondary-emission materials and photo- 
cathodes. Because of the superior 
statistics of gallium phosphide (one 
of the more recently developed sec- 
ondary-emission materials discussed 
in the section on Photoemission and 
Secondary Emission), a tight single- 
electron distribution curve can be 
obtained, as shown in Fig. 77. The 
tight distribution permits easy loca- 
tion of the pulse-height discriminator, 
as is particularly evident when some 



zg 


100 




A A ' 1 1 




2c 










Ou 










U.Z 


80 






- 


o- 










Si 


60 






- 








V/~.SINGLE ELECTRON^, 




40 




u RESOLUTION b40 , /\ 


_ 








(FWHM) \ 




— Z) 










<£ 


20 


— 1 




- 


-jo: 










u ui 










£o- 




J 1 


1 1 1 1 1 





12 3 4 5 6 7 

PULSE HEIGHT -PHOTOELECTRON EQUI- 
VALENTS 

Fig- 77 — Single-electron distribution 
curve taken with a photomultiplier hav- 
ing a gallium phosphide first dynode. 

of the signal pulses are originated 
by two or more photoelectrons leav- 
ing the cathode simultaneously. If 
the average number of photoelectrons 
leaving the photocathode per pulse 
were three, a pulse-height distribu- 
tion similar to that shown in Fig. 
78 would be obtained. Gallium phos- 
phide provides a higher signal-to- 
noise ratio than would conventional 
secondary-emitting materials such as 
BeO or Cs 3 Sb when used in the same 
tube. 

Photocathode developments in- 
clude ERMA (Extended Red Multi- 
Alkali), a semitransparent photo- 
cathode having a response to 900 



Application of Photomultipliers 




97 

tion of wavelength and require pho- 
tomultipliers having a broad spectral 
range. The results of measurements 
of the absorption characteristics of 
substances are frequently expressed 
in terms of optical density. This 
logarithmic method correlates with 
the way the human eye discriminates 
differences in brightness. 

The transmission density D is de- 
fined by 



12 3 4 5 6 

PULSE HEIGHT/ No. OF ELECTRONS 

pig, 78 — Pulse-height distribution ob- 
tained with a photomultiplier having a 
gallium phosphide first dynode. Light 
level is such that three photoelectrons 
per pulse time is the most probable 
number. 

nanometers, and several negative- 
electron-affinity materials. Perhaps 
the most significant of these ma- 
terials is indium gallium arsenide, 
whose threshold wavelength in- 
creases with increasing indium con- 
tent. Spectral-response curves for 
some of the more recent photo- 
cathodes are shown in the section on 
Basic Performance Characteristics. 

PHOTOMETRIC APPLICATIONS 

Photometry is concerned with the 
measurement of luminous intensity 
and other parameters related to the 
radiant energy which produces visual 
sensation. Photometric units and con- 
version from radiant power are dis- 
cussed in a later section of this 
Manual. Photometric measurements 
may be made either by visual com- 
parison or by more accurate photo- 
electric methods. This section dis- 
cusses some of the applications of 
photomultipliers in photometry. 

Spectrophotometry 

Spectrophotometers measure the 
optical density of materials as a func- 



p 
D = logio r§ = log 



(104) 



where P is the radiant flux incident 
upon a sample, P t is the radiant flux 
transmitted by a sample, and T is 
the transmission figure or P t /P - 
Density measurements are useful in 
various applications to films and 
other transparencies in addition to 
chemical analysis where concentra- 
tion of a solution is studied as a 
function of wavelength. 

Color-Balancing Photometry 

A color-balancing photometer is 
used to determine color balance and 
exposure times necessary to produce 
photographic color prints from color 
negatives. Such a device is shown in 
block diagram form in Fig. 79; it 
allows the matching of the relative 
proportions of red, blue, and green 
light transmitted by a production 
negative to those of a master color 
negative. As the first step in the 
matching process, the master nega- 
tive is used to make an acceptable 
print. This first print is produced 
through trial and error by measure- 
ment of the relative proportions of 
red, blue, and green light transmitted 
through a key area of the master 
negative; the key area usually con- 
sists of a flesh tone or a gray area. 
The amount of light transmitted is 



98 



RCA Photomultiplier Manual 



DENSITY/TIME 




0+ REGULATED 
HIGH 
VOLTAGE 
POWER 
SUPPLY 



Fig. 79 — Block diagram of a color-balancing photometer. 



a measure of the density. The ex- 
posure time using white light and 
the lens opening used to obtain the 
satisfactory print are noted. 

Next, an area of the production 
negative similar to that on the mas- 
ter negative is chosen and the rela- 
tive proportions of the red, blue, and 
green light transmitted are measured 
again. By use of magenta and yel- 
low correcting filters, the color trans- 
mitted by the production negative 
can be balanced with that of the 
master negative. When the produc- 
tion negative is used, the lens open- 
ing is adjusted so that the exposure 
time with white light is the same as 
it was for exposure of the master 
negative. When the values of color- 
correcting filters and exposure times 
thus determined are used, prints from 
the production negative can be ob- 
tained which are very nearly as good 



as those obtained with the master 
color negative. 

Most color-balancing photometers 
employ steady light sources and 
handle small-amplitude signals. Con- 
sequently, the photomultiplier used 
must have low values of dark current 
and good stability. The life expect- 
ancy of tubes used in this application 
is long because the small signal levels 
reduce the effects of fatigue which 
might otherwise adversely affect 
measurement accuracy and repeat- 
ability. Low-current operation also 
provides for linear operation where 
the anode current is proportional to 
the input flux over the range of trans- 
mission values measured. The in- 
tensity range of a color-balancing 
photometer, given in terms of the 
ratio of the radiant flux incident upon 
a sample to the radiant flux trans- 
mitted by a sample, is usually of the 



Application of Photomultipliers 



99 



order of 1000 to 1 or more. It must 
be remembered that as in most 
photomultiplier applications, the 
power supplies used must be capable 
of providing voltages sufficiently 
regulated and free from ripple to 
assure minimum variation in sensi- 
tivity with possible line-voltage varia- 
tion. 

Densitometry 

A second example of the use of 
the equivalent photoelectric method 
in photometry measurements is in 
densitometry, or the measurement of 
diffuse transmission density. 

Although techniques such as those 
employed in the color-balancing 
photometer may be used successfully 
to measure density over an intensity 
range of 10 or 100 to 1 (density 1 
to density 2), their use becomes in- 
creasingly difficult as the range is 
increased to 1000 to 10,000 to 1 
(density 3 to density 4) or more. The 
large dynamic ranges encountered in 
color-film processing place severe re- 
quirements upon the photomultiplier 
because it must be operated in a 
constant-voltage mode. Problems de- 
velop in this mode at high-density 
values at which the dark current may 
become a significant proportion of 
the signal current. The random na- 
ture of this dark current, which pre- 
cludes its being "zeroed out", may 
lead to output-signal instability. As 
a result of the type of operation 
needed to produce the dynamic range 
required in density measurements, 
the photomultiplier anode current is 
high at low density values. These 
high currents may result in excessive 
fatigue, and, depending upon the 
operating point, perhaps non-linear 
operation. As with the color-balanc- 
ing photometer, a well regulated low- 
ripple power supply is needed to 
assure accurate measurements. 



Logarithmic Photometry 

In the measurement of absorption 
characteristics, the changes in bright- 
ness levels vary over such a large 
range that it is advantageous to use 
a photometer whose response is ap- 
proximately logarithmic. This re- 
sponse enables the photometer to be 
equipped with a meter or read-out 
scale that is linear and provides pre- 
cise readings even at high optical 
densities. 

A simplified circuit of a logarith- 
mic photometer capable of measur- 
ing film density with high sensitivity 
and stability and of providing an 
appropriate logarithmic electrical re- 
sponse and linear meter indication of 
density over three or four density 
ranges is shown in Fig. 80. 

The circuit of Fig. 80, in which 
the photomultiplier operates at a 
constant current, minimizes photo- 
multiplier fatigue and eliminates the 
need for a regulated high-voltage 
supply. The feedback circuit illus- 
trated develops a signal across Rl 
proportional to the anode current. 
This signal controls the bias applied 
to the control tube and automatically 
adjusts the current in the voltage- 
divider network. By this means, the 
dynode voltage is maintained at a 
level such that the anode current is 
held constant at a value selected to 
minimize the effects of fatigue. At 
optical densities of 3, the dynode 
supply voltage may be 1000 volts; 
at optical densities of less than 1, 
the dynode voltage may be as low 
as 300 volts. Dynode voltage is trans- 
lated into density by means of the 
scale on voltmeter VI. 

Because there is an approxi- 
mately exponential variation of sensi- 
tivity of a photomultiplier with ap- 
plied voltage (as discussed in the 
section on Basic Performance Char- 
acteristics) in a constant-current 



100 



RCA Photomultiplier Manual 




Fig. 80 — Block diagram of a logarithmic photometer. 



mode, the voltage becomes a logarith- 
mic function of the input light flux 
and thus a linear measure of density. 
In photometric equipment, the re- 
lationship between dynode voltage 
and the logarithm of the incident 
radiant flux P„ is not quite linear 
and, consequently, some compensa- 
tion is required. This compensation 
is provided in the circuit of Fig. 80 
by the incorporation of a variable 
and automatic shunt across the volt- 
meter VI. As the density values in- 
crease, the effective value of the 
shunt resistance is reduced. This cir- 
cuit not only compensates for photo- 
multiplier non-linearity, but also for 
the non-linearity of the optical sys- 
tem employed so that the measure- 
ments obtained through its use will 
conform to American Standards 



Association standards for the de- 
termination of diffuse density. 

SPECTROMETRY 
APPLICATIONS 

Spectrometry, the science of spec- 
trum analysis, applies the methods of 
physics and physical chemistry to 
chemical analysis. Spectrometric ap- 
plications include absorption, emis- 
sion, Raman, solar, and vacuum 
spectrometry, and fluorometry. The 
uses made of photomultipliers in 
these applications are described in 
the following paragraphs. 

Absorption Spectrometry 

Absorption spectrometry, used to 
detect radiant energy in the visible, 



Application of Photomultipliers 

ultraviolet, and infrared ranges, is 
one of the most important of the in- 
strumental methods of chemical 
analysis. It has gained this import- 
ance largely as a result of the de- 
velopment of equipment employing 
photomultipliers as detectors. The 
principle underlying absorption spec- 
trometry is the spectrally selective 
absorption of radiant energy by a 
substance. The measurement of the 
amount of absorption aids the scien- 
tist in determining the amount of 
various substances contained in a 
sample. 

The essential components of an 
absorption spectrometer are a source 
of radiant energy, a monochromator 
for isolating the desired spectral 
band, a sample chamber, a detector 
for converting the radiant energy to 
electrical energy, and a meter to 
measure the electrical energy. The 
spectral ranges of the source and de- 
tector must be appropriate to the 
range in which measurements are to 
be made; in some cases this range 
may include one wavelength; in 
others, it may scan all wavelengths 
between the near-ultraviolet and the 
near-infrared. 

The side-on photomultiplier has, 
in general, been the most widely used 
in spectrometric applications. The 
side-on tube is relatively small and 
has a rectangularly shaped photo- 
cathode that is compatible with the 
shape of the light beam from an exit 
slit. Spectrometric applications re- 
quire tubes with good stability, high 
anode sensitivity, low dark current, 
and broad spectral sensitivity. 

Before the development of the new 
negative-electron-affinity type photo- 
emitters, such as gallium arsenide, 
it was necessary to use more than 
one detector in the measurement of 
radiant energy from the near-ultra- 
violet to the near-infrared part of 
the spectrum. A photomultiplier hav- 



101 

ing a gallium arsenide photoemitter 
can now be used to detect radiant 
energy from 940 nanometers to the 
cut-off point of the photomultiplier 
window in the near-ultraviolet. 

Raman Spectroscopy 

There are two types of molecular 
scattering of light, Rayleigh and 
Raman. Rayleigh scattering is the 
elastic collision of photons with 
the molecules of a homogeneous 
medium. Because the scattered pho- 
tons do not gain energy from or lose 
energy to the molecule, they have 
the same energy hvo as the incident 
photons. A classic example of Ray- 
leigh scattering of light from gas 
molecules is the scattering of the 
sun's light rays as they pass through 
the earth's atmosphere; the scattering 
accounts for the brightness and the 
blueness of the sky. 

Raman scattering is the inelastic 
collision of photons with molecules 
that produces scattered photons of 
higher or lower energy than the ini- 
tial photons. During the collision 
there is a quantized exchange of en- 
ergy which, depending on the state 
of the molecules, determines whether 
the initial photon gains or loses en- 
ergy. The differences in energy levels 
are characteristic of the molecule. 
If the excitation frequency is v l and 
the photon emitted after the inter- 
action has a frequency of i/ 2 , the 
interaction results in a change in the 
energy of an initial photon by hv , 
where hi/ = \hv 2 - to'il- When hv 2 
is greater than hi/ lt the initial pho- 
ton has gained energy from the mole- 
cule that was in the excited state; 
in the reverse case, the initial photon 
has given up energy to the molecule 
in the unexcited state. 

Early Raman instruments had a 
number of disadvantages and were 
difficult to use; it was difficult to 



102 

find a stable high-intensity light 
source and to discriminate against 
Rayleigh scattering of the exciting 
line. With the recent development of 
high-quality monochromators and 
the advent of the laser light source, 
a renewed interest in the Raman ef- 
fect as an analytical method of 
chemical analysis has taken place. 
Raman spectrophotometers are gen- 
erally used to investigate the struc- 
ture of molecules and to supplement 
other methods of chemical analysis, 
particularly infrared-absorption spec- 
trometry. 



RCA Photomultiplier Manual 

The scattered photons from a 
Raman interaction are so few in 
number that only the highest-quality 
photomultipliers can be used as de- 
tectors. The tube should have high 
collection efficiency, high gain, good 
multiplication statistics, low noise, 
and high quantum efficiency over the 
spectral range of interest. Fig. 81 
shows a Raman spectrum. To re- 
duce the effects of Rayleigh scatter- 
ing, a source of noise in Raman 
spectroscopy, the photomultiplier is 
placed at right angles to the light 
beam, whose wavelength has been 
chosen as long as possible within 
the range of interest. 



100 



1 1 i 1 1 1 1 1 i i i — i — i — i — r 



80- 



60 



40 




PENTENE-2-RAMAN SPECTRUM 



i i i — r 



t— r 



' WAVELENGTH (cm) ' WAVEN "MBER (cm"') 




-MINIUM I I 1 I I I i i I . i , 

4000 3000 2000 1500 



1000 



-I— I 1 1 I I l 



WAVENUMBER(cnT') 
Fig. 81 — Typical Raman spectrum. 



500 



cm ' 



103 



Voltage-Divider Considerations 



THE interstage voltage gradients 
for the photomultiplier elements 
may be supplied by individual volt- 
age sources; however, the usual 
source is a resistive voltage divider 
placed across a high-voltage source, 
as shown in Fig. 82. 



Rl 



^v- Lp 




t -Anode 
RL - Anode Lo«d 

Resistors 
DY n -Dynodi Stage 
R n - Voltage Divider 

Resistors 
n - Total Number 
DY n _l of Oynode Stages 

K -Cathode 



DYn-2 



RESISTANCE VOLTAGE DIVIDER 

The response of a 931 A photo- 
multiplier using a conventional volt- 
age divider, as shown in Fig. 82, 
with equal voltage per stage is shown 
in Fig. 83 as a function of the light 
flux input. 1 (The value of R L was 
essentially zero for this measure- 
ment.) The anode current is shown 
relative to the divider current at zero 



TYPE 93IA 

SUPPLY VOLTS ' 1000 




1 I I 



pig, 82 — Schematic diagram of a re- 
sistive voltage divider. 



0.4 0.8 1.2 1.6 20 24 
£ LIGHT FLUX - ARBITRARY UNITS 

Fig 83 — The relative response of a 
931A photomultiplier as a function of 
the light flux using the circuit of Fig. 
82 with equal stage voltage (at zero 
lieht level). Curve is taken from Fig. I 
of Ref. 1. 

light level. The superlinearity region 
is explained by the fact that the in- 
crease in dynode voltage resulting 
from the redistribution of the voltage 
loss across the anode-to-last-dynode 



104 



RCA Photomultiptier Manual 



divider resistor tends to reduce anode 
collection efficiency. The decrease in 
sensitivity which occurs beyond re- 
gion A of Fig. 83 results from the 
extension of voltage losses to the last 
two or three dynode resistors caus- 
ing defocusing and skipping in the 
associated dynode stages. In order 
to prevent this loss and assure a high 
degree of linearity, the current 
through the voltage-divider network 
should be at least ten times the maxi- 
mum average anode current required 
to prevent the dynode voltages from 
varying. In calculating the voltage- 
divider current, the average anode 
current must first be estimated; this 
estimate requires knowledge of the 
value of the input (light) signal and 
the required output (electrical) signal. 

VOLTAGE RATIO 

A resistance voltage divider must 
be designed to divide the applied 
voltage equally or unequally among 
the various stages as required by the 
electrostatic system of the tube. The 
most common voltage between stages 
is usually referred to as the stage 
voltage and the voltage between other 
stages as relative to the most com- 
mon stage voltage. 

The voltage distribution is nor- 
mally specified in the following way: 



Between 

K-D Vl 
D yi - Dy 2 
Dy„_! - Dy n 
Dy n — Anode 



Relative 
Voltage 

r (D 

F (2) 
r (n) 
r (n + l) 



where n represents the number of 
the dynode stage and r (J) (j = 1 to 
n+1) represents the relative inter- 
stage voltage. There is always one 
more interstage voltage than there 
are dynode stages. 



The following formula may be 
used to calculate the voltage be- 
tween stages: 



n + 1 

where r t = 2 r„ and Vj is the 

i=i 



voltage between the elements K — 
Dyi, Dyj — Dy 2 , and Dy n — Anode. 

VOLTAGE-DIVIDER RESISTORS 

The voltage-divider resistor values 
required for each stage can be de- 
termined from the value of the total 
resistance required of the voltage 
divider and the voltage-divider ratios 
of the particular tube type. The inter- 
stage resistance values are in propor- 
tion to the voltage-divider ratios as 
follows: 



Rj = Rt — 

rt 



where Rj is the resistance between 
elements Dy 3 and Dy j _ 1 . The rec- 
ommended resistance values for 
a photomultiplier voltage divider 
range from 20,000 ohms per stage to 
5 megohms per stage; the exact 
values are usually the result of a 
compromise. If low values of resist- 
ance per stage are utilized, the power 
drawn from the regulated power sup- 
ply may be excessively large. The 
resistor power rating should be at 
least twice the calculated rating to 
provide a safety margin and to pre- 
vent a shift in resistance values as 
a result of overheating. 

Photomutliplier noise or a shift in 
gain may result from heat emanating 
from the voltage-divider resistors; 
therefore, the divider network and 



Voltage-Divider Considerations 

other heat-producing components 
should be located so that they will 
not increase the tube temperature. 
Resistance values in excess of five 
megohms should be avoided because 
current leakage between the photo- 
multiplier terminals could cause a 
variation of the interstage voltage. 
The type of resistor used in a di- 
vider depends on the dynode struc- 
ture with which the divider will be 
used. Close tolerance resistors, such 
as the wire-wound types, are nor- 
mally required with the focused or in- 
line structures. On the other hand, 
interdynode voltages in the venetian- 
blind structures can vary widely with 
but little effect on the photomulti- 
plier. For this reason, the resistors 
used with a venetian-blind structure 
need only be of a less stable variety, 
such as carbon. 

CATHODE-TO-FIRST-DYNODE 
REGION 

The over-all performance of a 
tube can be improved by maintain- 
ing a high electric field in the cath- 
ode-to-first-dynode region to reduce 
the transit-time spread of photoelec- 
trons arriving at the first dynode and 
minimize the effect of magnetic fields. 
A high first-dynode gain, which im- 
plies a high cathode-to-first-dynode 
voltage, is particularly important in 
the detection of extremely low light 
levels where the statistics of photo- 
emission become important. 

A high cathode-to-first-dynode 
voltage of approximately 600 to 800 
volts is recommended in photomul- 
tipliers having a gallium phosphide 
first dynode. Otherwise, the higher 
secondary-emission capability of this 
material and its improved statistics 
are not utilized. 

It is frequently useful to control 
the gain of the photomutiplier by 
varying the over-all voltage. This ar- 



105 

rangement, however, may have the 
disadvantage of reducing the gain 
in the critical first stage. This disad- 
vantage can be avoided by using a 
zener diode to hold this critical volt- 
age constant. 

INTERMEDIATE STAGES 

In applications in which it is de- 
sirable to control the anode sensi- 
tivity without changing the over-all 
voltage, the voltage of a single dy- 
node may be varied. Fig. 84 shows 
the variation of anode current for 
a 931 A photomultiplier when one of 
the dynode voltages is varied while 
the total supply voltage is held con- 
stant. The dynode should be selected 
from the middle of the dynode string 
because a variation of dynode poten- 
tials near the cathode or anode 
would have a detrimental effect on 
photomultiplier operation. 



100 

8 



TYPE 93IA 

ANODE SUPPLY VOLTAGE =1000 V 



I I I I I I. 



ui 



o 

< 

UJ 

> 

< 




1 I I I I I ' I I I I I I I 



"500 600 700 800 

DYNODE N0.6-T0-CATH0DE VOLTS 

p i g< 84 — The output-current variation 

of a 931A when the voltage on one 

dynode (No. 6) is varied while the total 

anode voltage remains fixed. 



106 



RCA Photomultiplier Manual 



VOLTAGE DIVIDERS FOR 
PULSED OPERATION 

In applications in which the input 
signal is in the form of pulses, the 
average anode current can be de- 
termined from the average pulse am- 
plitude and the duty factor. The 
total resistance of the voltage-divider 
network is calculated for the average 
anode current for dc operation. 

In cases in which the average 
anode current is orders of magnitude 
less than a peak pulse current, dy- 
node potentials can be maintained 
at a nearly constant value during 
pulse durations of 100 nanoseconds 
or less by use of charge-storage ca- 
pacitors at the tube socket. The 
voltage-divider current need only be 
sufficient to provide the average 
anode current for the photomulti- 
plier; the high peak currents required 
during the large-amplitude light 
pulses may be supplied by the ca- 
pacitors. 

The capacitor values depend upon 
the value of the output charge asso- 
ciated with the pulse or train of 
pulses. The value of the final-dynode- 
to-anode capacitor C is given by 



100 ^ 



where C is in farads, q is the total 
anode charge per pulse in coulombs, 
and V is the voltage across the ca- 
pacitor. The factor 100 is used to 
limit the voltage change across the 
capacitor to a 1-per-cent maximum 
during a pulse. Capacitor values for 
preceding stages should take into ac- 
count the smaller values of dynode 
currents in these stages. Conserva- 
tively, a factor of approximately 2 
per stage is used. Capacitors are not 
required across those dynode stages 



at which the peak dynode current is 
less than 1/10 of the average cur- 
rent through the voltage-divider net- 
work. 



MEDIUM-SPEED 
PULSE APPLICATIONS 

In applications in which the out- 
put current consists of pulses of short 
duration, the capacitance C L of the 
anode circuit to ground becomes 
very important. The capacitance C L 
is the sum of all capacitances from 
the anode to ground: photomulti- 
plier-anode capacitance, cable ca- 
pacitance, and the input capacitance 
of the measuring device. For pulses 
with a duration much shorter than 
the anode time constant R L C L , the 
output voltage is equal to the product 
of the charge and 1/C L because the 
anode current is simply charging a 
capacitor. The capacitor charge then 
decays exponentially through the 
anode load resistor with a time con- 
stant of R L C L . To prevent pulses 
from piling up on each other, the 
maximum value of R L C L should be 
much less than the reciprocal of the 
repetition rate. 

An important example of this type 
of operation is scintillation counting, 
for example with NaI(Tl). The time 
constant of the scintillations is 0.25 
microsecond and, because the inte- 
gral of the output current pulse is 
a measure of the energy of the inci- 
dent radiation, the current pulse is 
integrated on the anode-circuit ca- 
pacitance for a period of about 1 
microsecond. To prevent pulse pile- 
up, the anode-circuit time constant 
is selected to be about 10 micro- 
seconds. Because the scintillations oc- 
cur at random, the maximum average 
counting rate is limited to about 
10 kHz. 



Voltage-Divider Considerations 
FAST-PULSE APPLICATIONS 

In fast-pulse light applications, it 
is recommended that the photomulti- 
plier be operated at negative high 
voltage with the anode at ground po- 
tential. A typical voltage-divider cir- 
cuit with series-connected capacitors 
used with a positive ground is shown 
in Fig. 85. The parallel configura- 
tion of capacitors may also be used, 



TO 

CATHODE 
-HIGH 
VOLTAGE 



DY, 
1 



107 

on a 50-ohm source and a 50-ohm 
load impedance; careful wiring tech- 
nique is required to assure the best 
impedance match. A good impedance 
match to the transmission line and 
anode of the tube is provided by 
capacitively coupling the last, and 
sometimes the next to last, dynode 
to the ground braid of the 50-ohm 
coaxial signal cable. 



DY n _ 3 DY n _ 2 t*n-l 



DY n 



^V\ArAAArAAAr»AAAr*- »AAAn *AAA/-i ►AAAh ►AA/V 



0.005,iF 0.005fiF O.OOI/iF 



Fig 85 — Series-connected capacitors in voltage-divider circuit using positive 
ground for pulse-light applications. 



as shown in Fig. 86. This circuit per- 
mits lower-inductance connections to 
the ground plane. The parallel ar- 
rangement requires capacitors of 
higher voltage ratings. Regardless of 
the configuration, the capacitors must 
be located at the socket. The capaci- 
tor arrangements just described may 
also be applied to negative-ground 
applications. 

The wiring of the anode is very 
critical in pulse applications if line- 
arity and pulse shape are to be pre- 
served. Most pulse circuitry is based 



Fig. 87 illustrates the use of by- 
pass capacitors (C BP ) on the anode 
signal line. Because of the bypass 
capacitors, the transmission-line ele- 
ments in the figure are viewed by a 
fast pulse as a single line between 
two ground planes. 

WIRING TECHNIQUES 

Good high-frequency wiring tech- 
niques must be employed in wiring 
photomultiplier sockets and asso- 
ciated voltage dividers if loss of time 



TO 

CATHODE 
-HIGH 
VOLTAGE 



DY, 



DY, 



3R 



LwVA/WAAArHW*- 



DY n _ 3 DY n _ 2 DY n _, DY n 

"ft'' 

R R R R 

-i ►AAAr' >AAA^ >AAAh ►WV-i 



0.005/iF 



0.005/^ 



0.001/iF 



Fig. 86 — Parallel-connected capacitors in voltage-divider circuit for 
pulsed-light application. 



108 



RCA Photomultiplier Manual 



ur B -|, 



LOW-INDUCTANCE 
STRIPS 



DYn-l 
POTENTIAL 



'ANODE 




DY„ 
POTENTIAL 



Fig. 87 — Bypass capacitors used to 
make the last two dynodes appear as 
a ground plane to a fast-pulsed signal. 



resolution in pulse operation is to 
be minimized. Fig. 88(a) illustrates 
the pulse shape obtained from an 
8575 photomultiplier excited by a 
0.5-nanosecond rise-time light pulse. 
The pulse shape is most easily seen 
with the aid of a repetitive light 
pulser and a sampling oscilloscope. 
Fig. 88(b) indicates the general type 
of distortion encountered with the 
use of improper charge-storage ca- 
pacitors, capacitors either too small 
in value or too inductive. The output 
pulse appears clipped in comparison 
to that of Fig. 88(a) and illustrates 
the ringing that may occur in an im- 
properly wired socket, a condition 
that worsens as pulse amplitude in- 
creases. 

Fig. 89 shows a socket wired for 
a negative high-voltage application. 
The disk-type bypass capacitors are 
mounted in series with minimum lead 
length because the self-inductance of 
a few millimeters of wire becomes 
critical in nanosecond-pulse work; 
care should also be taken in dressing 





Fig. 88 — (a) Pulse shape obtained from 
photomultiplier excited by a 0.5-nano- 
second-rise-time light pulse, and (b) 
distortion encountered with improper 
charge-storage capacitors and ringing 
in an improperly wired socket. 



the bypass capacitors and coaxial 
cable. The resistors for the voltage 
divider are shown mounted on the 
socket. In applications requiring 
minimum dark current, the resistors 
should be remote from the photo- 
multiplier to minimize heating ef- 
fects. In negative-high-voltage ap- 
plications, the socket should be 
mounted in a copper or brass co- 
axial cylinder to provide a suitable 
ground plane to which the capacitors 
can be connected; this construction 
is shown in Fig. 90. 



Voltage-Divider Considerations 



ii ii ii 
!' n n 
!! u ii 




SHORT LENGTH RG58V 
LEADING TO BNC OR 
GR847 CONNECTOR 



pig. 89 — Anode detail of socket wired 
for a negative high-voltage application. 



PHOTOMULTIPLIER 
SOCKET MACHINED 
TO FIT TUBING 




CHARGE STORAGE 
CAPACITORS CONNECTED 
TO GROUND CYLINDER 



pig. 90 — Tube socket mounted in 

copper or brass coaxial cylinder for 

negative-high-voltage applications. 



CHECKING SOCKET AND 
TUBE PERFORMANCE 

Some photomultipliers typically 
display a reflected-pulse rise time of 
the order of 1.5 nanoseconds for an 
initiating pulse of the order of 50 
picoseconds when the tube and socket 
are tested with a time-domain re- 
flectometer, the instrument used to 
test the anode-pin region of the 
socket. The socket should simulate 
an open circuit. If the output pulse 
can be viewed with a sampling oscil- 
loscope, the pulse shape can be in- 
spected for signs of clipping or ring- 
ing, effects more prominent at high 



109 

pulse amplitudes. Output-pulse am- 
plitude may be increased by increas- 
ing the tube voltage and may be 
measured with a multichannel ana- 
lyzer or an oscilloscope. 

The effectiveness of the charge- 
storage capacitors can be verified 
with a simple linearity test. The out- 
put of a photomultiplier varies as 
Va, where V is the applied voltage 
and alpha (a) is a constant; thus a 
plot of pulse amplitude as a function 
of the logarithm of the voltage value 
should yield a straight line. 

A two-pulse technique may be em- 
ployed to test linearity at constant 
operating voltage. This technique re- 
quires a means for attenuating the 
light signals by a factor of 0.5 to 
0.1; a filter in the light beam suf- 
fices. In applying the technique, the 
ratio of the amplitude of the two 
pulses is established at low pulse 
amplitudes where linear operation is 
assured. Next, the pulse amplitude 
is increased, either by increasing the 
tube voltage or by increasing the 
light intensity; a constant pulse-am- 
plitude ratio indicates linear opera- 
tion. 

TAPERED DIVIDERS 

Some applications require that 
photomultipliers sustain high signal 
currents for short time intervals, tens 
of nanoseconds or less. In general, 
photomultipliers are capable of sup- 
plying 0.2 ampere or more into a 
50-ohm load for short durations. 
However, the voltage divider must 
be tailored to the application to al- 
low a photomultiplier to deliver 
these high currents. 

The principal limitation on current 
output (into a 50-ohm system) is 
space charge at the last few stages. 
This space charge can be overcome 
if the potential difference across the 
last few stages is increased by use 



110 



RCA Photomultiplier Manual 



of a tapered divider rather than an 
equal-volts-per-stage divider. The 
tapered divider places 3 to 4 times 
the normal interstage potential dif- 
ference across the last stage. The 
progression leading to the 4-times 
potential difference should be grad- 
ual to maintain proper electrostatic 
focus between stages; a progression 
of 1, 1 ... 1, 1, 1.5, 2.0, 2.5, 3 is 
recommended. 

Anode signals 400 to 500 volts in 
amplitude can be obtained from 
photomultipliers. These large voltage 
excursions are often useful, for ex- 
ample, in driving electro-optical 
modulators. Because the photomulti- 
plier is nearly an ideal constant- 
current source, its output voltage 
into a high-impedance load is limited 
only by the potential difference be- 
tween the last dynode and the anode. 
If the potential difference is 100 
volts, the anode cannot swing 
through more than a 100-volt excur- 
sion. By impressing a much higher 
potential difference between the 
anode and last dynode by means of 
a tapered divider, greater voltage 
swings can be obtained. 

DYNAMIC COMPRESSION OF 
OUTPUT SIGNAL 

Most photomultipliers operate 
linearly over a dynamic range of 
six or seven orders of magnitude, a 
range few monitor devices can ac- 
commodate without requiring range 
changes. When compression of the 
dynamic range is desired, a logarith- 
mic amplifier is sometimes used. The 
photomultiplier may be operated in 
a compressed-output mode, how- 
ever, without the need for additional 
compression circuitry. 

The voltage excursion of the anode 
is limited by the potential difference 
between the last dynode and the 
anode (with no signal current flow- 



ing). If the potential difference is 
reduced from, for example, 100 volts 
to 10 volts, several orders of magni- 
tude can be compressed into a sin- 
gle decade. The exact relationship 
between input-light intensity and the 
compressed output voltage developed 
across a load resistor varies among 
tube types and voltage dividers, and 
must be determined empirically. 

CURRENT PROTECTION 
OF PHOTOMULTIPLIER 

If a photomultiplier is accidentally 
exposed to an excessive amount of 
light, it may be permanently dam- 
aged by the resultant high currents. 
To reduce this possibility, the re- 
sistive voltage-divider network may 
be designed to limit the anode cur- 
rent. The average anode current of 
a photomultiplier cannot much ex- 
ceed the voltage-divider current; 
therefore, the voltage-divider net- 
work serves as an overload protection 
for the tube. If overexposure is ex- 
pected frequently, interdynode cur- 
rents which can be quite excessive, 
may cause loss of gain. In some ap- 
plications it may be worth while to 
protect against dynode damage by 
using resistors in series with each 
dynode lead. 1 

HIGH-VOLTAGE SUPPLY 

The recommended polarity of the 
photomultiplier power-supply volt- 
age with respect to ground depends 
largely on the application intended. 
Of course, the cathode is always 
negative with respect to the anode; 
however, in some pulsed applications, 
such as scintillation counting, the 
cathode should be grounded and the 
anode operated at a high positive 
potential with a capacitance-coupled 
output. The scintillator and any mag- 
netic or electrostatic shields should 



Voltage-Divider Considerations 



111 



also be connected to ground poten- 
tial to eliminate leakage currents 
from ground to the photocathode and 
to prevent the shock hazard caused 
by the scintillator being at high volt- 
age. 

In applications in which the signal 
cannot be passed through a coupling 
capacitor, the positive side of the 
power supply should be grounded. 
The cathode is then at a high nega- 
tive potential with respect to ground. 
When this arrangement is used, extra 
precautions must be taken in the 
mounting and shielding of the photo- 
multiplier. If a potential gradient 
exists across the tube wall, scintilla- 
tions occurring in the glass will in- 
crease dark noise. If this condition 
exists for a sufficiently long period 
of time, the photocathode will be 
permanently damaged by the ionic 
conduction through the glass. To 
prevent this situation when a shield 
is used, the shield is connected to 
photocathode potential. Light-shield- 
ing or supporting materials used in 
photomultipliers must limit leakage 
currents to 10- 12 ampere or less. 
Fig. 91 shows a curve of the effect 
of external-shield potential on photo- 
multiplier noise. 

To reduce the shock hazard to 
personnel, a very high resistance 
should be connected between the 
shield and the negative high voltage. 



1 1 1 1 1 1 1 1 

- TUBE TEMPERATURE = 22°C 


T 


o w 


— 




2 ZS 


s 


- 


Si 


^s^ 




io- |2 §S 


, — """^ 




8 11 


_ 


- 




6 W 


Ill 


l._l 


4 



-IO0O -800 -600 / -400 -200 
EXTERNAl'sHIELD volts 
RELATIVE TO ANODE VOLTS 

Fig. 91 — Curve of effect of external- 
shield potential on noise in a 
photomultiplier. 

POWER-SUPPLY STABILITY 

The output signal of a photomulti- 
plier is extremely sensitive to varia- 
tions in the supply voltage. There- 
fore, the power supply should be very 
stable and exhibit minimum ripple. 
As an approximate "rule of thumb", 
power-supply stability (in per-cent 
variation) should be at least ten 
times less than the maximum output- 
current variation (in per cent) that 
can be tolerated. 

REFERENCES 

1. R. W. Engstrom and E. Fischer, 
"Effects of Voltage-Divider Char- 
acteristics on Multiplier Photo- 
tube Response", Rev. Sci. Instr., 
Vol. 28, p. 525 (1957). 



112 



Photometric Units and 
Photometric-to-Radiant 
Conversion 



PHOTOMETRY is concerned with 
the measurement of light; the 
origins of the photoelectric industry 
were associated with the visible spec- 
trum. It was appropriate, therefore, 
that the units for evaluating photo- 
sensitive devices be photometric. To- 
day, however, although many of the 
applications of photosensitive de- 
vices are for radiation outside the 
visible spectrum, for many purposes 
the photometric units are still re- 
tained. Because these units are based 
on the characteristics of the eye, it 
is appropriate that the introduction 
to this discussion begin with a con- 
sideration of some of the character- 
istics of the eye. 

CHARACTERISTICS OF 
THE EYE 

The sensors in the retina of the 
human eye are of two kinds: "cones" 
which predominate the central (or 
foveal) vision and "rods" which pro- 
vide peripheral vision. The cones are 
responsible for our color vision; rods 
provide no color information but in 
the dark-adapted state are more sen- 
sitive than the cones and thus pro- 
vide the basis of dark-adapted vision. 
Because there are no rods in the 
foveal region, faint objects can more 
readily be observed at night when 
the eye is not exactly directed toward 



the faint object. The response of the 
light-adapted eye (cone vision) is re- 
ferred to as the Photopic eye re- 
sponse; the response of the dark- 
adapted eye (rod vision) is referred 
to as Scotopic eye response. 

Although characteristics of the 
human eye vary from person to per- 
son, standard luminosity coefficients 
for the eye were defined by the Com- 
mission Internationale d'Eclairage 
(International Commission on Il- 
lumination) in 1931. These standard 
CLE. luminosity coefficients, listed 
in Table II, refer to the Photopic 
eye response. They represent the 
relative luminous equivalents of an 
equal-energy spectrum for each 
wavelength in the visible range, as- 
suming foveal vision. An absolute 
"sensitivity" figure established for 
the standard eye relates photometric 
units and radiant power units. At 555 
nanometers, the wavelength of maxi- 
mum sensitivity of the eye, one watt 
of radiant power corresponds to 680 
lumens. 

For the dark-adapted eye, the peak 
sensitivity increases and is shifted 
toward the violet end of the spectrum. 
A tabulation of the relative scotopic 
vision is given in Table III. The peak 
luminosity for scotopic vision occurs 
at 511 nanometers and is the equiva- 
lent of 1746 lumens/ watt. Fig. 92 
shows the comparison of the absolute 



Photometric Units and Photometric-to-Radiant Conversion 113 

Table II — Standard Luminosity Coefficient Values. 



Wmltwtli 


CLE. 


(Milometers) 


VllM 


380 


0.00004 


390 


0.00012 


400 


0.0004 


410 


0.0012 


420 


0.0040 


430 


0.0116 


440 


0.023 


450 


0.038 


460 


0.060 


470 


0.091 


480 


0.139 


490 


0.208 


500 


0.323 



(■uometers) 

510 
520 
530 
540 
550 
560 
570 
580 
590 
600 
610 
620 
630 



CLE. 
Value 

0.503 
0.710 
0.862 
0.954 
0.995 
0.995 
0.952 
0.870 
0.757 
0.631 
0.503 
0.381 
0.265 



Wevoknfth 
(ninemeten) 

640 
650 
660 
670 
680 
690 
700 
710 
720 
730 
740 
750 
760 



CLE. 
Vain 

0.175 

0.107 

0.061 

0.032 

0.017 

0.0082 

0.0041 

0.0021 

0.00105 

0.00052 

0.00025 

0.00012 

0.00006 



luminosity curves for scotopic and 
photopic vision as a function of 
wavelength. 

The sensitivity of the eye outside 
the wavelength limits shown in 
Tables II and III is very low, but 
not actually zero. Studies with in- 
tense infrared sources have shown 
that the eye is sensitive to radiation 
of wavelength at least as long as 1050 
nanometers. Fig. 93 shows a com- 
posite curve given by Griffin, Hub- 
bard, and Wald * for the sensitivity of 
the eye for both foveal and peripheral 
vision from 360 to 1050 nanometers. 
According to Goodeve 2 the ultra- 
violet sensitivity of the eye extends 
to between 302.3 and 312.5 nano- 
meters. Below this level the absorp- 
tion of radiation by the proteins of 
the eye lens apparently limits further 
extension of vision into the ultra- 
violet. Light having a wavelength of 
302 nanometers is detected by its 
fluorescent effect in the front part of 
the eye. 

PHOTOMETRIC UNITS 

Luminous intensity (or candle- 
power) describes luminous flux per 
unit solid angle in a particular direc- 



tion from a light source. The measure 
of luminous intensity is the funda- 
mental standard from which all other 




300 



400 500 600 700 

WAVELENGTH-NANOMETERS 



Pi g% 92 — Absolute luminosity curves 
for scotopic and photopic eye response. 



114 RCA Photomultiplier Manual 

Table III — Relative Luminosity Values for Photopic and Scotopic Vision. 



WAVELENGTH 


PHOTOPIC VX 


SCOTOPIC VX 


(nm) 


(B > 3 cd m -») 


(B < 3 1 10-« cd m->) 


350 





0.0003 


360 


— 


0.0008 


370 


— 


0.0022 


380 


0.00004 


0.0055 


390 


0.00012 


0.0127 


400 


0.0004 


0.0270 


410 


0.0012 


0.0530 


420 


0.0040 


0.0950 


430 


0.0116 


0.157 


440 


0.023 


0.239 


450 


0.038 


0.339 


460 


0.060 


0.456 


470 


0.091 


D.576 


480 


0.139 


0.713 


490 


0.208 


0.842 


500 


0.323 


0.948 


510 


0.503 


0.999 


520 


0.710 


0.953 


530 


0.862 


0.849 


540 


0.954 


0.697 


550 


0.995 


0.531 


560 


0.995 


0.365 


570 


0.952 


0.243 


580 


0.870 


0.155 


590 


0.757 


0.0942 


600 


0.631 


0.0561 


610 


0.503 


0.0324 


620 


0.381 


0.0188 


630 


0.265 


0.0105 


640 


0.175 


0.0058 


650 


0.107 


0.0032 


660 


0.061 


0.0017 


670 


0.032 


0.0009 


680 


0.017 


0.0005 


690 


0.0082 


0.0002 


700 


0.0041 


0.0001 


710 


0.0021 




720 


0.00105 




730 


0.00052 




740 


0.00025 


— 


750 


0.00012 




760 


0.00006 




770 


0.00003 


- 



Photometric Units and Photometric-to-Radiant Conversion 115 



photometric units are derived. The 
standard of luminous intensity is the 
candela; the older term candle is 
sometimes still used, but refers to the 
new candle or candela. 

The candela is defined by the 
radiation from a black body at the 
temperature of solidification of plati- 
num. A candela is one-sixtieth of the 
luminous intensity of one square 
centimeter of such a radiator. 




300 500 700 900 IIOO 

WAVELENGTH — NANOMETERS 

Fig. 93 — Relative spectral sensitivity 

of the dark-adapted foveal and 

peripheral retina. 

A suitable substandard for practi- 
cal photoelectric measurements is the 
developmental-type calibrated lamp, 
RCA Dev. No. C70048, which oper- 
ates at a current of about 4.5 amperes 
and a voltage of 7 to 10 volts. A 
typical lamp calibrated at a color 
temperature of 2854°K provides a 
luminous intensity of 55 candelas. 
The luminous intensity of a tungsten 
lamp measured in candelas is usually 
numerically somewhat greater than 
the power delivered to the lamp in 
watts. (The photoelectric industry is 
in the process of changing the color- 
temperature standard for the tungsten 
test lamp. A color temperature of 



2870 °K has served as the basic test 
standard for photoelectric measure- 
ments in this country for about 30 
years. In order to provide a more 
universal standard, most industries 
and government laboratories are 
changing to 2854 °K color tempera- 
ture. This latter temperature is in 
common use in Europe. Illuminant 
A as designated by the CLE. is a 
tungsten lamp operated at 2854°K 
color temperature. The text of this 
Manual utilizes the 2854°K figure, 
although tube data are still based on 
2870 °K. The difference, however, is 
generally negligible. 3 Test lamps may 
be obtained with either temperature 
calibration.) 

Luminous flux is the rate of flow 
of light energy, that characteristic of 
radiant energy which produces visual 
sensation. The unit of luminous flux 
is the lumen, which is the flux emitted 
per unit solid angle by a uniform 
point source of one candela. Such a 
source produces a total luminous flux 
of 4>r lumens. 

A radiant source may be evaluated 
in terms of luminous flux if the 
radiant-energy distribution of the 
source is known. If W(X) is the total 
radiant power in watts per unit wave- 
length, total radiant power over all 
wavelengths is f co W(A) dA., and the 
total luminous flux L in lumens can 
be expressed as follows: 

L = 680 f °°W(A) y(A) dA. 

where y(X) represents the luminos- 
ity coefficient as a function of wave- 
length. At a wavelength of 555 nano- 
meters, one watt of radiant power 
corresponds to 680 lumens. The 
lumen is the most widely used unit 
in the rating of photoemissive de- 
vices; for photomultipliers the typi- 
cal test levels of luminous flux range 
from 10~ 7 to 10- 5 lumen (0.1 to 10 
microlumens). 



116 RCA Photomultiplier Manual 

Table IV — Luminance Values for Some Common Sources. 



Source Luminance 

(FooUamberts) 

Sun, as observed from Earth's surface at meridian 4.7 X 10 8 

Moon, bright spot, as observed from Earth's surface 730 

Clear blue sky 2300 

Lightning flash 2 X 10 10 

Atomic fission bomb, 0.1 millisecond after firing, 90-feet diameter ball 6 X 10" 

Tungsten filament lamp, gas-filled, 16 lumen/watt 2.6 X 10 6 

Piain carbon arc, positive crater iJX 10 6 

Fluorescent lamp, T-12 bulb, cool white, 430 mA, medium loading 2000 



Illuminance (or illumination) is the 

density of luminous flux incident on 
a surface. A common unit of illumi- 
nation is the footcandle, the illumi- 
nation produced by one lumen 
uniformly distributed over an area 
of one square foot. It follows that a 
source of one candela produces an 
illumination of one footcandle at a 
distance of one foot. 

Table IV lists some common values 
of illumination encountered in photo- 
electric applications. Further infor- 
mation concerning natural radiation 
is shown in Fig. 94, which indicates 
the change in natural illumination at 
ground level during, before, and 
after sunset for a condition of clear 
sky and no moon. 4 

Photometric luminance (or bright- 
ness) is a measure of the luminous 
flux per unit solid angle leaving a 
surface at a given point in a given 
direction, per unit of projected area. 
The term photometric luminance is 
used to distinguish a physically 
measured luminance from a subjec- 
tive luminance. The latter varies 
with illuminance because of the shift 
in spectral response of the eye 
toward the blue region at lower levels 
of illuminance. The term luminance 




-4-3-2-1 O I 2 3 

HOURS BEFORE AND AFTER SUNSET 

Fig. 94 — Natural illuminance on the 

earth for the hours immediately before 

and after sunset with a clear sky and 

no moon. 

describes the light emission from a 
surface, whether the surface is self- 
luminous or receives its light from 
some external luminous body. 

For a surface which is uniformly 
diffusing, luminance is the same re- 
gardless of the angle from which the 



Photometric Units and Photometric-to-Radiant Conversion 



117 



surface is viewed. This condition re- 
sults from the fact that a uniformly 
diffusing surface obeys Lambert's law 
(the cosine law) of emission. Thus, 
both the emission per unit solid angle 
and the projected area are propor- 
tional to the cosine of the angle be- 
tween the direction of observation 
and the surface normal. 

A logical unit of luminance based 
on the definition given above is a 
candela per unit area. When the unit 
of area is the square meter, this unit 
is called a nit; when the unit of area 
is a square centimeter, the unit is a 
stilb. It is also possible to refer to a 
candela per square foot. However, 
none of these units is as commonly 
used in photoelectric measurement as 
the footlambert, a unit of photo- 
metric luminance equal to 1/77 
candela per square foot. The advan- 
tage of using the footlambert for a 
uniform diffuser is that it is equiva- 
lent to a total emission of one lumen 
per square foot from one side of the 
surface. This relationship can be 
demonstrated by the following con- 
ditions, as shown in Fig. 95. An ele- 
mentary portion of a diffusing sur- 
face having an area of A square feet 
has a luminance of one footlambert, 
or 1/7T candela per square foot. The 
light flux of interest is that emitted 
into an elementary solid angle, 2ir 



sin0 d6. At an angle of 6, the projec- 
tion of the elementary area is equal 
to A cos 6. Because the luminous 
flux in a particular direction is equal 
to the product of the source strength 
in candelas and the solid angle, the 
total luminous flux L in lumens from 
the area A may be obtained by inte- 
gration over the hemisphere as 
follows: 



Jr/2 



1 




Fig. 95 — Diagram illustrating Lam- 
bert's law and the calculation of total 
luminous flux from a diffuse radiator. 



(A cos 0) 
(27rsin0)d0 = A (1.6) 



In other words, the total flux from 
a uniform diffuser having a lumi- 
nance of one footlambert is one 
lumen per square foot. 

An advantage of the above rela- 
tionship is that the illumination at a 
surface in front of and parallel to an 
extended and uniformly diffusing sur- 
face having a luminance of one foot- 
lambert is equal to one lumen per 
square foot or one footcandle. As 
a result, an instrument reading il- 
luminance in footcandles indicates 
photometric luminance or brightness 
in footlamberts if the instrument is 
illuminated essentially from the entire 
hemisphere. (This statement neglects 
the possible perturbation caused by 
the measurement instrument.) 

In a typical application, a uni- 
fortnly diffusing radiating surface 
may be of such small size that it can 
be considered practically a point 
source. However, if the radiator is 
assumed to be a flat surface radiat- 
ing according to Lambert's law, the 
distribution of flux about the point 
is not the same as for an ordinary 
point source. In this case, if the sur- 
face luminance is one footlambert 
and the area is A square foot, the 
flux per steradian in a direction 
normal to the surface would be 



118 



RCA Photomultiplier Manual 



A(1/tt) lumens, or at an angle 6 
with respect to the normal line the 
flux would be A(l/7r) (cos 0) lumens 
per steradian. 

All photometric data in this 



Manual are presented in units of 
candelas, lumens, footcandles, and 
footlamberts; Table V is a conver- 
sion table for various photometric 
units. 



Table V — Conversion Table for Various Photometric Units. 

Luminous Intensity 

1 candela (cd) = 1 lumen/steradian (lm sn" 1 ) 

Luminous Flux 

4b- lumens = total flux from uniform point source of 1 candela. 

Illuminance 

1 footcandle (fc) = 1 lumen/ft 2 

= 10.764 lux (meter candle) 
(lumen/meter 2 ) 

= 0.001076 phot (lumen/cm s ) 

Luminance 

1 footlambert (fL) = \U candela/ft 2 

= 0.0010764 lambert (1/r candela/cm 2 ) 

= 1.0764 millilamberts 

= 3.426 nits (candela/meter 2 ) 

= 0.0003426 stilbs (candela/cm 2 ) 

= 0.3183 candela/ft 2 

= 10.764 apostilbs (!/*• candela/meter 2 ) 



3. 



REFERENCES 

D. R. Griffin, R. Hubbard, and G. 
Wald, "The Sensitivity of the Hu- 
man Eye to Infrared Radiation," 
JOSA, Vol. 37, No. 7 (1947). 
C. F. Goodeve, "Vision in the 
Ultraviolet," Nature (1934). 
R. W. Engstrom and A. L. More- 
head, "Standard Test Lamp Tem- 
perature for Photosensitive De- 
vices — Relationship of Absolute 



and Luminous Sensitivities," RCA 
Review, Vol. 28 (1967). 
4. I.E.S. Lighting Handbook, Illumi- 
nating Engineering Society, New 
York, New York (1959). 
R. Kingslake, Applied Optics and 
Optical Engineering, Vol. 1, Chap- 
ter 1, Table II, Academic Press, 
New York and London (1965). 
J. W. Walsh, Photometry, Con- 
stable and Co., Ltd., London 
(1953). 



5. 



6. 



119 



Radiant Energy and Sources 



RADIANT energy is energy travel- 
ing in the form of electromag- 
netic waves; it is measured in joules, 
ergs, or calories. The rate of flow 
of radiant energy is called radiant 
flux; is it expressed in calories per 
second, in ergs per second, or, prefer- 
ably, in watts (joules per second). 

BLACK-BODY RADIATION 

As a body is raised in tempera- 
ture, it first emits radiation primarily 
in the invisible infrared region. Then, 
as the temperature is increased, the 
radiation shifts toward the shorter 
wavelengths. A certain type of radia- 
tion called black-body radiation is 
used as a standard for the infrared 
region; other sources may be de- 
scribed in terms of the black body. 

A black body is one which absorbs 
all incident radiation; none is trans- 
mitted and none is reflected. Because, 
in thermal equilibrium, thermal radia- 
tion balances absorption, it follows 
that a black body is the most effi- 
cient thermal radiator possible. A 
black body radiates more total power 
and more power at a particular 
wavelength than any other thermally 
radiating source at the same tempera- 
ture. Although no material is ideally 
black, the equivalent of a theoretical 
black body can be achieved in the 
laboratory by providing a hollow 
radiator with a small exit hole. The 



radiation from the hole approaches 
that from a theoretical black radia- 
tor if the cross-sectional area of the 
cavity is large compared with the 
area of the exit hole. The character- 
istic of 100-per-cent absorption is 
achieved because any radiation enter- 
ing the hole is reflected many times 
inside the cavity. 

The radiation distribution for a 
source which is not black may be 
calculated from the black-body radia- 
tion laws provided the emissivity as 
a function of wavelength is known. 
Spectral emissivity is defined as the 
ratio of the output of a radiator at 
a specific wavelength to that of a 
black body at the same temperature. 
Tungsten sources, for which tables 
of emissivity data are available, 1 are 
widely used as practical standards, 
particularly for the visible range. 
Tungsten radiation standards for the 
visible range are frequently given 
in terms of color temperature, in- 
stead of true temperature. The color 
temperature of a selective radiator is 
determined by comparison with a 
black body: when the outputs of the 
selective radiator and a black body 
are the closest possible approxima- 
tion to a perfect color match in the 
range of visual sensitivity, the color 
temperature of the selective radiator 
is numerically the same as the black- 
body true temperature. For a tung- 
sten source, the relative distribution 



120 

of radiant energy in the visible spec- 
tral range is very close to that of a 
black body although the absolute 
temperatures differ. However, the 
match of energy distribution becomes 
progressively worse in the ultraviolet 
and infrared spectral regions. 

SOURCE TYPES 

Tungsten lamps are probably the 
most important type of radiation 
source because of their availability, 
reliability, and constancy of oper- 
ating characteristics. Commercial 
photomultiplier design has been con- 
siderably influenced by the character- 
istics of the tungsten lamp. A relative 
spectral-emission characteristic for a 
tungsten lamp at 2854 °K color tem- 
perature is shown in Fig. 96. 



TUNGSTEN LAMP 2854 *K (STANDARD TEST) 



RCA Photomultiplier Manual 



g FLUORESCENT LAMP-DAYLI6HT 
H60r^ 





800 1600 2400 3200 

WAVELENGTH— NANOMETERS 
Fig. 96 — Relative spectral-emission 
characteristic for a tungsten lamp at a 
color temperature of 2854' K. 

The common fluorescent lamp, a 

very efficient light source, consists of 
an argon-mercury glow discharge in 
a bulb internally coated with a phos- 
phor that converts ultraviolet radia- 
tion from the discharge into useful 
light output. There are numerous 
types of fluorescent lamps, each with 
a different output spectral distribu- 
tion depending upon the phosphor 
and gas filling. The spectral response 
shown in Fig. 97 is a typical curve 
for a fluorescent lamp of the day- 
light type. 



300 800 600 700 
WAVELENGTH-NANOMETERS 
Fig. 97 — Typical spectral-emission 
curve for a daylight-type fluorescent 
lamp. 

A very useful point source 2 is 
the zirconium concentrated-arc lamp. 

Concentrated-arc lamps are available 
with ratings from 2 to 300 watts, and 
in point diameters from 0.003 to 
0.116 inch. Operation of these lamps 
requires one special circuit to pro- 
vide a high starting voltage and an- 
other well filtered and ballasted cir- 
cuit for operation. 

Although many types of electrical 
discharge have been used as radiation 
sources, probably the most important 
are the mercury arc and the carbon 
arc. The character of the light 
emitted from the mercury arc varies 
with pressure and operating condi- 
tions. At increasing pressures, the 
spectral-energy distribution from 
the arc changes from the typical mer- 
cury-line spectral characteristic to 
an almost continuous spectrum of 
high intensity in the near-infrared, 
visible, and ultraviolet regions. Fig. 
98 shows the spectral-energy distri- 
bution from a water-cooled mercury 
MERCURY LAMP 
40r 




400 S00 600 700 

WAVELENGTH— NANOMETERS 
Fig. 98 — Typical spectral-emission 
curve for a water-cooled mercury-arc 
lamp at a pressure of 130 atmospheres. 



Radiant Energy and Sources 



121 



arc at a pressure of 130 atmospheres. 
The carbon arc is a source of great 
intensity and high color temperature. 
A typical energy-distribution spec- 
trum of a dc high-intensity arc is 
shown in Fig. 99. Figs. 100 and 101 
show relative spectral-emission char- 
acteristic curves for xenon and argon 
arcs; Table VI specifies typical 
parameters for all sources described. 



CARBON-ARC LAMP 
8p— i — r— r 



ARGON- ARC LAMP 




300 400 S00 600 700 
WAVELENGTH- NANOMETERS 

Fig. 99 — Typical spectral-emission 
curve for a dc high-intensity carbon- 
arc lamp. 

In recent years the development of 
various types of lasers and p-n light- 
emitting diodes with very high modu- 
lation frequencies and short rise times 
has increased the types of sources 
that photomultipliers are called upon 

XENON -ARC LAMP 
60| 1 1 1 1 — r 




400 600 800 1000 1200 

WAVELENGTH - NANOMETERS 

Fig. 100 — Typical spectral-emission 
curve for a xenon-are lamp. 





200 



400 600 800 1000 1200 
WAVELENGTH - NANOMETERS 



1400 



Fig. 101 — Typical spectral-emission 
curve for an argon-arc lamp. 



to detect. Although many of these 
interesting devices have their prin- 
cipal wavelengths of emission in the 
infrared beyond the sensitivity range 
of photomultiplier tubes, some do 
not. Because of the growing impor- 
tance of laser applications and the use 
of photomultipliers for detecting 
their radiation, Tables VII through 
XI are provided as reference data 
on crystalline, gas, and liquid lasers, 
and on p-n junction light-emitting 
diodes. 

LIGHT SOURCES FOR TESTING 

Monochromatic sources of many 
wavelengths may be produced by 
narrow-band filters or monochroma- 
tors. Narrow-band filters are more 
practical for production testing, but, 
at best, such tests are time-consuming 
and subject to error. Monochromatic 
sources are not used in general- 
purpose testing because most appli- 
cations involve broader-band light 
sources; a monochromatic test might 
grossly misrepresent the situation be- 
cause of spectral-response variations. 

A broad-band source is probably 
more useful as a single test because 
it tends to integrate out irregularities 
in the spectral-response characteris- 
tic and more nearly represents the 



122 



RCA Photomultiplier Manual 



Table VI — Typical Parameters for the Most Commonly Used Radiant 

Energy Sources. 



LAMP TYPE 


DC 

INPUT 

POWER 

(WATTS) 


ARC 

DIMENSIONS 

(»■) 


LUMINOUS 
FLUX 
<!■> 


LUMINOUS AVERAGE 

EFFICIENCY LUMINAKCE 

(IbW-1) <€*■»-») 


Mercury Short Arc 
(high pressure) 


200 


2.5X1.8 


9500 


47.5 


250 


Xenon Short Arc 


150 


1.3X1.0 


3200 


21 


300 


Xenon Short Arc 


20,000 


12.5X6 


1,150,000 


57 


3000 

(in 3 mm 

X6mm) 

100 

1400 


Zirconium Arc 

Vortex-Stabilized 
Argon Arc 


100 
24,800 


1.5 
(diam ) 

3X10 


250 
422,000 


2.5 
17 


Tungsten 

Light 

Bulbs 


f 10 

100 

[ 1,000 


- 


79 

1630 

21,500 


7.9] 
16.3 
21.5 


10 
to 
25 


Fluorescent Lamp 
Standard Warm White 


40 


— 


2,560 


64 





Carbon Arc 
Non-Rotating 


2,000 


=5X5 


36,800 


18.41 


175 

to 

800 


Rotating 


15,800 


«8X8 


350,000 


22.2J 


Sun 


- 


- 


- 


- 


1600 


Table VII — Typical Characteristics of a 

Systems. 


Number of Useful Crystal Laser 


HOST 


DOPANT 


WAVELENGTH 

OF LASER 

(m"1) 


MODE AND HIGHEST 
TEMPERATURE OF 
OPERATION (°K) 



AI203 

AI203 

MgF2 
MgF2 
ZnF2 
CaW04 

CaF2 
CaMoC-4 
Y3AI5O12 
LaF3 



0.05% 
Cr 3+ 
0.5% 
Cr 3 * 

Ni*+ 

1J S 
Co*+ 

Co? + 

1% 
Nd 3+ 

Nd* + 

1.8% 
Nd'+ 

Nd 3+ 

1%, 
Nd 5+ 



0.6934 
0.6929 
0.7009 
0.7041 
0.7670 
1.6220 

1.7500 
1.8030 
2.6113 

1.0580 
0.9145 
1.3392 
1.0460 

1.0610 

1.0648 

1.0633 



CW.P350 
P300 


P77 
P77 
P300 


P77 


P77 
P77 


P77 


CW300 
P77 
P300 


P77 


CW300 


CW360 
P440 


P300 



Radiant Energy and Sources 



123 



Table VII — Typical Characteristics of a Number of Useful Crystal Laser 

Systems, (cont'd.) 







WAVELENGTH 


MODE AND HI6HEST 


HOST 


DOPANT 


OF LASER 


TEMPERATURE OF 






(c"») 


OPERATION (°K) 


LaF3 


Pr X+ 


0.5985 


P77 








CaW04 


0.5% 

p r 3+ 


1.0468 


P77 


Y2O3 


5% 
Eu 3+ 


0.6113 


P220 








CaF2 


Ho 3+ 


0.5512 


P77 


CaW04 


05% 
Ho 3+ 


2.0460 


P77 


Y3AI5O12 


Ho 3 + 


2.0975 


CW77 
P300 


CaW04 


1% 
E f 3+ 


1.6120 


P77 


Ca(Nb03)2 


Er 3 + 


1.6100 


P77 


Y3AI5O12 


Er 3+ 


1.6602 


P77 


CaW04 


Tm 3+ 


1.9110 


P77 


Y3AI5O12 


Tm 3+ 


2.0132 


CW77 
P300 


Er203 


Tm 3+ 


1.9340 


CW77 


Y3AI5O12 


Yb 3+ 


1.0296 


P77 


CaF2 


0.05% 


2.6130 


P300 




U 3+ 




CW77 


SrF2 


U 3 + 


2.4070 


P90 


CaF2 


.01% 
Srri 2 * 


0.7083 


P20 








SrF2 


.01% 
Sm 2+ 


0.6969 


P4.2 


CaF2 


.01% 


2.3588 


CW77 




Dy 2+ 




P145 


CaF2 


0.01% 


1.1160 


P27 




Tm 2+ 




CW4.2 



Table VIII — Comparison of Characteristics of Continuous Crystalline 

Lasers. 



MATERIAL 
ACTIVE SYSTEM 



SENSITIZER 



OPTICAL PUMP 



LENGTH EFF <» 
(Mm) 



POWER 
(WATTS) 



OPERATING 
TEMP. (°K) 



Dy 2+ CaF2 





W 


2.36 


0.06 


1.2 


77 


Cr 3+ Al203 


— 


Hg 


0.69 


0.1 


1.0 


300 


Nd 3+ Y3Al50i2 


— 


W 


1.06 


0.2 


2 


300 








1.06 


0.6 


15 


300 


Nd 3+ Y3Al50i2 


— 


Plasma Arc 


1.06 


0.2 


200 


300 


Nd 3 +Y3Al50l2 


— 


Na Doped Hg 


1.06 


0.2 


0.5 


300 


Nd3+Y3Al50i2 


Cr 3 + 


Hg 


1.06 


0.4 


10 


300 


Ho 3 +Y3Al50l2 


[Er 3 +, Yb 3+ , Tm 3+ ] 


W 


2.12 


5.0 


15 


77 



124 



RCA Photomultiplier Manual 



typical application. The tungsten 
lamp has been used for many years 
because it is relatively simple, stable, 
and inexpensive, and maintains its 
calibration. The tungsten lamp emits 
a broad band of energy with rela- 
tively smooth transitions from one 
end of the spectrum to the other. 
Its principal disadvantage as a gen- 
eral source is its lack of ultraviolet 
output and relatively low blue out- 
put. 



Sources such as arcs and glow dis- 
charges are difficult to calibrate and 
show serious time variations. 

Filters are frequently used to nar- 
row the spectral range for specific 
purposes; however, they sometimes 
contribute to errors because of sig- 
nificant transmission outside the 
band of interest. Filters are also sub- 
ject to change in transmission with 
time and are very difficult to repro- 
duce with identical characteristics. 



Table IX — Typical Characteristics of p-n Junction Light-emitting Diodes. 



CRYSTAL 


WAVELENGTH 
0>»> 


LASER 
ACTION 


PbSe 


8.5 


Yes 


PbTe 


6.5 


Yes 


InSb 


5.2 


Yes 


PbS 


4.3 


Yes 


In As 


3.15 


Yes 


(ln x Gai_ x )As 


0.85-3.15 


Yes 


ln(P,As!_ x ) 


0.91-3.15 


Yes 


GaSb 


1.6 


No 


InP 


0.91 


Yes 


GaAs 


0.90 


Yes 


Ga(A Sl _ x P x ) 


0.55-0.90 


Yes 


CdTe 


0.855 


No 


(Zn x Cd!_ x )Te 


0.59-0.83 


No 


CdTe-ZnTe 


0.56-0.66 


No 


BP 


0.64 


No 


Cu 2 Se-ZnSe 


0.40-0.63 


No 


Zn (Se x Te j_ x ) 


0.627 


No 


ZnTe 


0.62 


No 


GaP 


0.565 


No 


GaP 


0.68 


No 


SiC 


0.456 


7 



Radiant Energy and Sources 125 

Table X — Typical Characteristics of a Number of Useful Gas Lasers. 



gas 



Ne 
(ionized) 

Ne 
(unionized) 

He-Ne 
(unionized) 



Xe 
(ionized) 

Xe 
(unionized) 



A 
(ionized) 



N2 

(ionized) 

Kr 
(ionized) 

C02 

(molecular 
excitation) 

CF3I 

H2O 

(molecular 

excitation) 

CN 

(molecular 

excitation) 



PRINCIPAL 

WAVELENGTHS 

(cm) 



OUTPUT POWER 



0.3324 



0.5401 



337 



TYPICAL 



MAXIMUM 



0.5944-0.6143 
0.6328 
1.1523 
3.3913 


10 ^W 

lmW 

1-5 mW 

<lmW 


0.4603-0.6271 
0.5419-0.6271 
0.4965-0.5971 


5mW 

10 mW 

lmW 


2.026 
3.507 
5575 
9007 


lmW 

0.5 mW 
0.5 mW 


4880 
05145 
04545 to 
05287 


0.5 W 
0.5 W 
1.5 W 


033 


{ = 


0.3507 

0.5208-0.6871 

0.5682 


- 


10.552 I 

10.572 

10.592 




50 W 


1.315 


— 


27.9 
118 
118 




- 



3.2 kW 

10 kW 

1.2 W 
lmW 
10 M W 

50 mW 



PULSED OR 
CONTINUOUS 



10 mW 
10 mW 


Pulsed 
CW 


lkW 


Pulsed 


150 mW 
25 mW 
10 mW 


Pulsed 
CW 
CW 
CW 


1W 
1W 


Pulsed 

CW 

Pulsed 


10 mW 

1 mW 

5mW 

5mW 


CW 
CW 
CW 
CW 


5W 

5W 

40 W 


CW 
CW 
CW 


200 kW 
100 mW 


Pulsed 
CW 


300 mW 

3 W 
100 mW 


CW 
CW 
CW 



CW 

Pulsed 

Pulsed 
Pulsed 
CW 

Pulsed 



Table XI — Typical Characteristics for Two Liquid Lasers. 



LIQUID 



PRINCIPAL 
WAVELENGTH 



PULSE ENERGY 
(PULSEWIDTH) 



Eu(0-Cl BTFA)4DMA* 
Nd +3 :SeOCl2" 



0.61175 
1.056 



0.1J 
10J(150pS) 



* dimethylammonium salt of tetrakis 
europium - ortho - chloro - benzoyltriflu- 
oracetonate 



* * trivalent neodymium in selenium 
oxychloride 



126 



RCA Photomultiplier Manual 



As a result of the work of indus- 
trial committees, virtually the entire 
photosensitive-device industry in the 
United States uses the tungsten lamp 
at 2870 °K color temperature* as a 
general test source. The lamp is cali- 
brated in lumens and is utilized in 
the infrared spectrum as well as the 
visible. Typical as well as maximum 
and minimum photosensitivities are 
quoted in microamperes per lumen. 

The principal disadvantages of us- 
ing the tungsten lamp as an industry 
standard test are that it does not pro- 
vide a direct measure of radiant 
sensitivity as a function of wave- 
length and that it is a somewhat mis- 
leading term when the response of 
the photomultiplier lies outside the 
visible range. To assist the scientist 
in using photomultipliers, technical 
specifications for RCA photomulti- 
plier types include photocathode 
spectral-response curves which give 
the sensitivity in absolute terms such 
as amperes per watt and quantum 
efficiency as a function of wave- 
length. Methods of computing the re- 
sponse of a given photodetector to 
a particular radiation source are out- 
lined in the following section on 
Spectra] Response and Source-Detec- 
tor Matching. 

FUNCTIONAL TESTING 

In many applications it is appro- 
priate to test photomultipliers in the 
same manner in which they are to be 
utilized in the final application. For 
example, photomultipliers to be 
used in scintillation counting may be 



tested by means of an NaI(Tl) crys- 
tal and a Cs 137 source or a simulated 
Nal light source utilizing a tungsten 
lamp whose light passes through a 
one-half stock-thickness Corning 
5113 glass-type CS-5-58 filter. Inter- 
ference-type filters are becoming in- 
creasingly important in isolating spe- 
cific wavelengths for testing photo- 
multiplier tubes for laser applications. 

TESTING FOR SCIENTIFIC AND 
MULTIPURPOSE APPLICATIONS 

When a photomultiplier is manu- 
factured for a variety of purposes, 
including scientific applications, it 
would be highly desirable if sensi- 
tivity were specified by a complete 
spectral response in terms of quan- 
tum efficiency or radiant sensitivity. 
This information could then be uti- 
lized in conjunction with the known 
spectral emission of any source to 
compute the response of the photo- 
multiplier to that source. Complete 
spectral sensitivity data, however, 
are rarely provided because it is un- 
necessary for most practical situa- 
tions and would considerably increase 
device costs. 

REFERENCES 

1. J. C. DeVos, Physica, Vol. 20 
(1954). 

2. W. D. Buckingham and C. R. 
Deihert, "Characteristics and Ap- 
plications of Concentrated Arc 
Lamps," Journal of the Society 
of Motion Picture Engineers, Vol. 
47 (1946). 



* Now being changed to 2854 °K color 
temperature to agree with international 
standards. 



127 



Spectral Response and 
Source-Detector Matching 



THIS section covers the signifi- 
cance of the spectral response of 
photomultiplier tubes, describes some 
of the methods used for measuring 
this response; and discusses in some 
detail the calculations and other con- 
siderations useful for matching the 
radiation source and the photomul- 
tiplier tube type for a specific appli- 
cation. 

SPECTRAL-RESPONSE 
CHARACTERISTICS 

A spectral-response characteristic 
is a display of the response of a 
photosensitive device as a function of 
the wavelength of the exciting radia- 
tion. Such curves may be on an ab- 
solute or a relative basis. In the lat- 
ter case the curves are usually nor- 
malized to unity at the peak of the 
spectral-response curve. For a photo- 
cathode the absolute radiant sensitiv- 
ity is expressed in amperes per watt. 
Curves of absolute spectral response 
may also be expressed in terms of 
the quantum efficiency at the particu- 
lar wavelength. If the curve is pre- 
sented in terms of amperes per watt, 
lines of equal quantum efficiency may 
be indicated for convenient reference. 
Typical curves are usually included 
in tube data bulletins. Because there 
are variations in spectral response 
from tube to tube, the typical spec- 
tral-response characteristic usually 



includes an indication of the range 
over which the peak of the curve may 
be expected to vary, as well as an 
indication of the range over which 
the cut-off at either end of the spec- 
tral response may vary. These cut- 
off points are usually expressed at 
10 per cent of the maximum value. 
The spectral-response characteris- 
tic, and particularly the long-wave- 
length cut-off, are dependent upon 
the chemical composition and the 
processing of the particular photo- 
cathode. On the other hand, the 
ultraviolet cut-off characteristic is 
determined primarily by the charac- 
teristic cut-off transmission of the 
window of the tube. 

MEASUREMENT TECHNIQUES 

In order to determine a spectral- 
response characteristic, a source of 
essentially monochromatic radiation, 
a current-sensitive instrument to 
measure the output of the photo- 
cathode, and a method of calibrating 
the monochromatic radiation for its 
magnitude in units of power are re- 
quired. 

The source of monochromatic 
radiation is often a prism or a grat- 
ing type of monochromator. Inter- 
ference filters may also be used to 
isolate narrow spectral bands. Al- 
though interference filters do not 
provide the flexibility of a mono- 



128 



RCA Photomultiplier Manual 



chromator, they may be indicated in 
situations in which repeated measure- 
ments are required in a particular re- 
gion of the spectrum. 

The width of the spectral transmis- 
sion band in these measurements 
must be narrow enough to delineate 
the spectral-response characteristic 
in the required detail. However, for 
the most part, spectral-response 
characteristics do not require fine 
detail and generally have broad peaks 
with exponential cut-off characteris- 
tics at the long-wavelength limit and 
rather sharp cut-offs at the short- 
wavelength end. For spectral mea- 
surements, therefore, a reasonably 
wide band is used. Such a band has 
the following important advantages: 
(1) because the level of radiation is 
higher, measurements are easier and 
more precise; and (2) spectral leak- 
age in other parts of the spectrum is 
relatively less important. Spectral 
leakage is a problem in any mono- 
chromator because of scattered radia- 
tion, and in any filter because there 
is some transmission outside the de- 
sired pass band. A double mono- 
chromator may be used and will 
greatly reduce the spectral leakage 
outside the pass band. The double 
monochromator is at a disadvantage 
in cost and complexity. If a pass band 
of 10 nanometers is used, spectral 
leakage can be insignificant for most 
of the spectral measurements. At the 
same time, this pass band is narrow 
enough to avoid distortion in the 
measured spectral-response charac- 
teristic. It is often advisable to vary 
the pass band depending upon what 
part of the curve is being measured. 
For example, at the long-wavelength 
cut-off where the response of the 
photocathode may be very small, the 
leakage spectrum may play a rather 
important part; thus it is advisable 
to increase the spectral bandpass of 
the measurement. Wide pass-band 



color filters which exclude the wave- 
length of the measurement and in- 
clude the suspected spectral leakage 
region, or vice versa, are used in 
checking the magnitude of the pos- 
sible leakage spectrum. 



ENERGY SOURCES 

Various radiant-energy sources are 
used to advantage in spectral-re- 
sponse measurements. A tungsten 
lamp is useful from about 450 nano- 
meters to beyond 1200 nanometers 
because of its uniform and stable 
spectral-emission characteristic. The 
linear type of filament also provides 
good optical coupling to the slits 
used in monochromators. A mercury- 
vapor discharge lamp provides a high 
concentration in specific radiation 
lines and thus minimizes the back- 
ground scattered-radiation problem. 
The mercury lamp is particularly 
useful in the ultraviolet end of the 
spectrum where the tungsten lamp 
fails. 

MEASUREMENT OF 
RADIANT POWER OUTPUT 

A radiation thermocouple or 
thermopile with a black absorbing 
surface is commonly used to measure 
the radiation power output at a spe- 
cific wavelength. Although these de- 
vices are relatively low in sensitivity, 
they do provide a reasonably reliable 
means of measuring radiation inde- 
pendent of the wavelength. The limi- 
tation to their accuracy is the flatness 
of the spectral absorption character- 
istic of the black coating on the de- 
tector. Throughout the visible and 
near-infrared regions there is usually 
no problem. There is some question, 
however, as to the flatness of the 
response in the ultraviolet part of 
the spectrum. 



Spectral Response and Source-Detector Matching 



129 



The output of the thermocouple 
or thermopile is a voltage propor- 
tional to the input radiation power. 
This voltage is converted by means 
of a suitable sensitive voltmeter to 
a calibrated measure of power in 
watts. The calibration may be ac- 
complished by means of standard 
radiation lamps obtained from the 
National Bureau of Standards. It is 
theoretically possible to calculate the 
monochromatic power from a knowl- 
edge of the emission characteristic 
of the source, the dispersion charac- 
teristic of the monochromator or the 
transmission characteristic of the fil- 
ter, and from the transmission char- 
acteristic of the various lenses. This 
procedure is difficult, subject to 
error, and is not recommended ex- 
cept perhaps in the case of a tung- 
sten lamp source combined with a 
narrow-band filter system. 

MEASUREMENT OF 
PHOTOCATHODE OUTPUT 

For measuring the spectral char- 
acteristic of a photocathode, a very 
sensitive ammeter is required. When 
the output of the photocathode of 
a photomultiplier is measured, the 
tube is usually operated as a photo- 
diode by connecting all elements 
other than the photocathode together 
to serve as the anode. When the 
photomultiplier is operated as a con- 
ventional photomultiplier, the output 



is very easy to measure. It is neces- 
sary, however, to be careful to avoid 
fatigue effects which could distort 
the spectral-response measurement. 
It should be noted that the spectral 
response of a photomultiplier may 
be somewhat different than that of 
the photocathode alone because of 
the effect of initial velocities and 
their effect on collection efficiency at 
the first dynode and because of the 
possibility of a photoeffect on the 
first dynode by light transmitted 
through the photocathode, especially 
if the first dynode is a photosensitive 
material such as cesium-antimony. 

A diagram of a typical arrange- 
ment for measurement of spectral re- 
sponse is shown in Fig. 102. In this 
arrangement a mirror is used in two 
angular orientations to reflect the 
radiation alternately to the thermo- 
couple and to the photocathode. 
Other methods are also used to split 
the monochromatic beam so that 
both thermocouple and photocathode 
can be sampled alternately or simul- 
taneously. 

SOURCE AND DETECTOR 
MATCHING 

One of the most important para- 
meters to be considered in the selec- 
tion of a photomultiplier type for a 
specific application is the photo- 
cathode spectral response. The spec- 
tral response of RCA photomultiplier 



MERCURY 
SOURCE 



SHUTTFR - M °NO- 
SHUTTtK CHRQMATOR 



VOLT- 
METER 



I THERMO- 
I COUPLE 



—\ 



TUBE 



PHOTO- _ BATTERY 
' 45-90V 






AM- 
METER 



I I MONO- 



TUNGSTEN 
SOURCE 



Fig. 102 — Diagram of typical arrangement for spectral-response measurements. 



130 



RCA Photomultiplier Manual 



tubes covers the spectrum from the 
ultraviolet to the near-infrared re- 
gion. In this range there are a large 
variety of spectral responses to 
choose from. Some cover narrow 
ranges of the spectrum while others 
cover a very broad range. The data 
sheet for each photomultiplier type 
shows the relative and absolute spec- 
tral-response curves for a typical 
tube of that particular type. The 
relative typical spectral-response 
curves published may be used for 
matching the detector to a light 
source for all but the most exacting 
applications. The matching of detec- 
tor to source consists of choosing the 
photomultiplier tube type that has a 
spectral response providing maximum 
overlap of the spectral distributions 
of detector and light source. 

MATCHING CALCULATIONS 

The average power radiating from 
a light source may be expressed as 
follows: 

P = P J.°W(A) dX (105) 

where P„ is the incident power in 
watts per unk wavelength at the peak 
of the relative spectral radiation 
characteristic, W(\), which is normal- 
ized to unity. 

If the absolute spectral distribu- 
tion for the light source and the ab- 
solute spectral response of the photo- 
multiplier tube are known, the 
resulting photocathode current I k 
when the light is incident on the de- 
tector can be expressed as follows: 






Ik = <r P„ / W (X) R (X) dX (106) 

where <r is the radiant sensitivity of 
the photocathode in amperes per 
watt at the peak of the relative curve, 
and R(X) represents the relative 



photocathode spectral response as a 
function of wavelength normalized 
to unity at the peak. When Eq. (106) 
is solved for the peak power per 
unit wavelength, P , and this solu- 
tion is substituted into Eq. (107), the 
cathode current is expressed as fol- 
lows: 



r 



J W(X) R(X) dX 
I k = (T p-L_ (1Q7) 



f 



W(X) dX 



The ratio of the dimensionless inte- 
grals can be defined as the matching 
factor, M. The matching factor is the 
ratio of the area under the curve de- 
fined by the product of the relative 
source and detector spectral curves 
to the area under the relative spec- 
tral source curve. 



M = 



/' 



W(X) R(X) dX 






(108) 



W(X) dX 



Fig. 103 shows an example of the 
data involved in the evaluation of 
the matching factor, M, as given in 
Eq. (108). 

If the input light distribution in- 
cident on the detector is modified 
with a filter or any other optical de- 
vice, the matching-factor formulas 
must be changed accordingly. If the 
transmission of the filter or optical 
device is f(\), the matching factor 
will be 



M = 



*f n 



W(X) R(X) fX dX 






W(X) f(X) dX 



Table XII shows a number of match- 
ing factors for various light sources 



Spectral Response and Source-Detector Matching 



131 



Table XII — Spectral Matching Factors.** 





















OTHER 










PHOTOCATHODES 






DETECTORS 




Pho- 
topic 


Sco- 
topic 






SI 


S4 


S10 


su 


S17 


S20 


S25 


•»• 




Notn 


r. 


k 


k 


k 


k 


k 


k 


1 


in 


Phosphors 






















PI 


a 


0.278 


0.498 


0.807 


0.687 


0.892 


0.700 


0.853 


0.768 


0.743 


P4 


a,b 


0.310 


0.549 


0.767 


0.661 


0.734 


0.724 


0.861 


0.402 


0.452 


P7 


a 


0.312 


0.611 


0.805 


0.709 


0.773 


0.771 


0.882 


0.411 


0.388 


Pll 


a 


0.217 


0.816 


0.949 


0.914 


0.954 


0.877 


0.953 


0.201 


0.601 


P15 


a 


0.385 


0.701 


0.855 


0.787 


0.871 


0.802 


0.904 


0.376 


0.495 


P16 


a 


0.830 


0.970 


0.853 


0.880 


0.855 


0.902 


0.922 


0.003 


0.042 


P20 


a 


0.395 


0.284 


0.612 


0.427 


0.563 


0.583 


0.782 


0.707 


0.354 


P22B 


c 


0.217 


0.893 


0.974 


0.960 


0.948 


0.927 


0.979 


0.808 


0.477 


P22G 


c 


0.278 


0.495 


0.807 


0.686 


0.896 


0.699 


0.855 


0.784 


0.747 


P22R 


c 


0.632 


0.036 


0.264 


0.055 


0.077 


0.368 


0.623 


0.225 


0.008 


P24 


a 


0.279 


0.545 


0.806 


0.696 


0.827 


0.725 


0.869 


0.540 


0.621 


P31 


a,d 


0276 


0.533 


0.811 


0.698 


0.853 


0.722 


0.868 


0.626 


0.651 


Nal 


e 


0.534 


0.923 


0.885 


0.889 


0.889 


0.900 


0.933 


0.046 


0.224 


Lamps 






















2870/2854 std 


f 


0.516* 


0.046* 


0.095* 


0.060* 


0.072* 


0.112* 


0.227* 


0.071** 


0.040* 


Fluorescent 





0.395 


0.390 


0.650 


0.496 


0.575 


0.635 


0.805 


0.502 


0.314 


Sun 






















In space 


h 


0.535* 


0.308* 


0.388* 


0.328* 


0.380* 


0.406* 


0.547* 


0.179* 


0.172* 


+2 air masses 


h,i 


0.536* 


0.236* 


0.348* 


0.277* 


0.315* 


0.360* 


0.513* 


0.197* 


0.175* 


Day sky 


i 


0.537* 


0.520* 


0.556* 


0.508* 


0.589* 


0.581* 


0.700* 


0.170* 


0.218* 


Black bodies 






















6000°K 





0.533* 


0.308* 


0.376* 


0.320* 


0.375* 


0.397* 


0.521* 


0.167* 


0.159* 


3000°K 





0.512* 


0.053* 


0.102* 


0.067* 


0.080* 


0.120* 


0.232* 


0.075* 


0.044* 


2870°K 





0.504* 


0.044* 


0.090* 


0.057* 


0.069* 


0.106* 


0.216* 


0.067* 


0.038* 


2854°K 





0.500* 


0.042* 


0.088* 


0.055* 


0.068* 


0.103* 


0.211* 


0.065* 


0.037* 


2810°K 





0.493* 


0.039* 


0.081* 


0.051* 


0.062* 


0.097* 


0.150* 


0.061* 


0.034* 


2042°K 


- 


0.401* 


0.008* 


0.023* 


0.011* 


0.014* 


0.033* 


0.090* 


0.018* 


0.007* 



* Entry valid only for 300-1200-nm wavelength in- 
terval. 
t For the total wavelength spectrum this entry would 

be 0.0294. 
Notes: a Registered spectral distribution. Data 
extrapolated as required. 
b Sulfide type. 
c RCA data. 
1 Low brightness type, 
e Harshaw Chemical Co. data. 
f Standard test lamp distribution. 
g General Electric Co. data. 
a From Handbook of Geoiiysics. 



i Approximately noon sealevel flux at 60° 
latitude. 

J From Gates between 300 nm and 530 nm. 
A 12,000°K blackbody spectral distribu- 
tion was assumed between 530 nm and 
1200 nm. 

k Registered spectral distribution. Data 
extrapolated as required. 

I Standard tabulated photopic visibility dis- 
tribution. 

n Standard tabulated scotopic visibility dis- 
tribution. 

** Data from E. H. Eberhardt, "Source-Detector 
Spectral Matching Factors", Applied Optics, 
7, 2037, (1968) 



132 



RCA Photomultiplier Manual 



IOO 


- 


I I 


i i 


1 1 


8 

6 

4 






f~\ 


RELATIVE - 
TUNGSTEN 
"LIGHT 
SOURCE - 
W(X)2854°K 




- 


/ l 1 


\ 


\ 


2 


"/ 


i 
i 


RELATIVE 
SENSITIVITY 
OF PHOTO - 
— CATHODE 
r(X) 


\\ 
\\ 
\\ 


IO 
8 




-WU)r(X) 


1 ~~ 


6 


i' 
i 


4 




i 




| - 






// 




ll ~~ 


2 

J 


I 


/ 

i 

ii i 


i 1 


f _ 

1 1 1 



300 



500 700 900 

WAVELENGTH — NANOMETERS 



Fig. 103 — Graphic example of factors 

used in evaluation of matching factor 

M. 

and spectral response characteris- 
tics. When the spectral range of a 
source exceeded 1200 nanometers, 
the integration was terminated at 
this wavelength. Because none of the 
photoresponses exceed 1200 nano- 
meters, conclusions as to the rela- 
tive merit of various combinations 
are still valid. 

When M is substituted for the in- 
tegral ratio in Eq. (107), the photo- 
cathode current becomes 



Ik = <r P M 



(109) 



Matching Factor 



In any photomultiplier applica- 
tion it is desirable to choose a detec- 



tor having a photocathode spectral 
response that will maximize the 
photocathode current, I k , for a given 
light source. Maximizing the cathode 
current is important to maximize the 
signal-to-noise ratio. From Eq. (109) 
it can be seen that the product of 
the matching factor M and the peak 
absolute photocathode sensitivity cr 
must be maximized to maximize the 
cathode current. It is advantageous 
then to define the product of the peak 
absolute cathode sensitivity cr and the 
matching factor M as N, a value to 
be selected as a maximum for a given 
light source: 



Therefore, 



N = o-M 



I k = PN 



(110) 



(I") 



The importance of taking into ac- 
count the absolute photocathode 
sensitivity, as well as the matching 
factor, is illustrated by a comparison 
of the S-l and S-20 photocathodes 
with a tungsten light source operat- 
ing at a color temperature of 
2854 °K. The S-l and the S-20 match- 
ing factors are 0.516 and 0.112, re- 
spectively. From the matching factors 
alone it appears that the S-l is the 
best choice of photocathode spectral 
response. The S-l has a peak abso- 
lute sensitivity of 2.5 milliamperes 
per watt and the S-20 has a peak 
absolute sensitivity of 64 milli- 
amperes per watt. Calculation of the 
variable N for the two photocathodes 
yields the following results: 

N (S-l) = 0.0025 x 0.516 = 0.00129 
N (S-20) = 0.064 x 0.112 = 0.00716 

These calculations show that the S-20 
photocathode will provide a substan- 
tially larger response to the tungsten 
lamp than the S-l. 



Spectral Response and Source-Detector Matching 133 

REFERENCES 

1. E. H. Eberhardt, "Source-Detec- Applied Optics, Vol. 7, p. 2037 

tor Spectral Matching Factors," (1968). 



134 



Technical Data 



THE tabulated data on the follow- 
ing pages are believed to be ac- 
curate and reliable for the listed types 
at the date of printing. However the 
data, especially for developmental 
types, are subject to change. It is 
recommended that RCA distributors 



or RCA field sales representatives be 
contacted for current information on 
the individual types. 

The data shown have been mea- 
sured using a tungsten-filament lamp 
operated at a color temperature of 
2870°K. It is intended that future 



PRELIMINARY SELECTION GUIDES 

For Photomultiplier Tubes by Spectral Response 





Nominal 

Tube 

Diamiter 

Inchts 


Number 

of 
States 


Viewlni 
Contifii- 
ration b 


Cage 
Struc- 
ture' 


RCA 
Typo 
No.* 


Outline, 
Basinj 
Diagram 


MAXIMUM 
RATINBS 


Spectral 

Response" 


Supply 

Voltata 

(E) 

V 


Averaga 

Anode 

Current 

mA 




3/4 


10 


H 


1 


C70102B 


4 


1500 


0.01 




1-1/8 


9 


S 


C 


C31004A 


11 


1500 


0.01 


101 (S-l) 


1-1/2 


10 


H 


C 


7102 


18 


1500 


0.01 




1-1/2 


10 


H 


C 


C70114D 


19 


1500 


0.01 




2 


12 


H 


1 


C70007A 


25 


2000 


0.01 




1/2 


9 


S 


C 


8571 


1 


1250 


0.02 




1-1/8 


9 


S 


C 


1P21 


11 


1250 


0.1 




1-1/8 


9 


S 


c 


931A 


11 


1250 


1.0 




1-1/8 


9 


S 


c 


4471 


11 


1250 


1.0 




1-1/8 


9 


S 


c 


4472 


11 


1250 


1.0 


102 (S-4) 


1-1/8 


9 


S 


c 


4473 


11 


1250 


0.1 




1-1/8 


9 


S 


c 


6328 


12 


1250 


0.1 




1-1/8 


9 


S 


c 


6472 


13 


1250 


0.1 




1-1/8 


9 


S 


c 


7117 


12 


1250 


0.1 




1-1/8 


9 


S 


c 


C7075J 


11 


1250 


0.1 


103 


1-1/8 


9 


S 


c 


1P28/V1 


11 


1250 


0.5 


1-1/8 


9 


S 


c 


1P28A/V1 


11 


1250 


0.5 



Technical Data 



135 



data for RCA photomultipliers be 
measured using the international 
color temperature of 2854°K. 

Variants of the listed photomulti- 
pliers are also available having elec- 
trostatic and magnetic shielding and/ 
or integral voltage-divider networks. 



Photomultiplier variants of each of 
the listed types are also available 
with permanently attached bases, 
with temporary bases attached to 
semiflexible leads, or with flexible 
leads only. 



TYPICAL CHARACTERISTICS AT THE 
SPECIFIED SUPPLY V0LTA6E AND 22° C 






Sensitivity 






Current 
Amplification 
(Approx.) 


Anode Dark 
Current nA@ 
Anode Luminous 
SonHMtyA/lm 




Supply 

Volute 
V 




Radiant 


Luminous 


Spectral 


Cathode 
mA/W 


Anode 
A/W 


Cathode 

uA/lm 


Anode 

A/lm 


Response* 


1250 


2.8 


310 


30 


3.3 


1.1 X 10 5 


800® 4 




1250 


1.9 


235 


20 


2.5 


1.25 X 10 5 


300® 2 




1250 


2.8 


660 


30 


7 


2.3 x 10 5 


1900@4 


101 (S-l) 


1250 


2.8 


420 


30 


4.5 


1.5 x 10» 


1700 ©4 




1250 


2.8 


940 


30 


10 


3.3 x 10 5 


400@4 




1000 


34 


73,000 


35 


75 


2.1 x 10 6 


2® 20 




1000 


40 


120,000 


40 


120 


3X10 6 


1@20 




1000 


40 


83,000 


40 


80 


2X10 6 


5 ©20 




1000 


40 


100,000 


40 


100 


2.5 X 10" 


5 ©20 




1000 


40 


100,000 


40 


100 


2.5 X 10" 


5® 20 




1000 


40 


160,000 


40 


160 


4xl0» 


1 ©20 


102 (S-4) 


1000 


- 


35,000 


- 


35 


- 


- 




1000 


- 


32,500 


- 


35 


- 


- 




1000 


- 


35,000 


- 


35 


- 


- 




1000 


40 


83,000 


40 


80 


2 x10 s 


5® 20 




1000 


48 


160,000 


60 


200 


3.3 x 10* 


2@40 


103 


1000 


48 


160,000 


60 


200 


3.3 X 10» 


2@40 



136 



RCA Photomultiplier Manual 



PRELIMINARY SELECTION GUIDES 

For Photomultiplier Tubes by Spectral Response (cont'd) 



Spectral 



Nominal 
Tube 

Diameter 
Inches 



Number Yiewinf Cap RCA 

of Configti- Struc- Type 

Steps ration b tore" No.* 



MAXIMUM 
RATINGS 



Outline, Supply Avorap 

Basini Voltan Anode 

Dlajram (E) Current 

V mA 



106 (S-10) 



107 (S-ll) 



1/2 



C70129H 



104 (S-5) 1-1/8 



1P28 



11 



1-1/8 



1P28A 



11 



105 (S-8) 1-1/8 



1P22 



11 



10 



6217 



26 



1-1/2 



10 



14 



2060 



18 



6810A 



32 



1250 



1250 



1250 



1250 



1250 



1250 



1-1/2 


10 


H 


C 


C70132B 


20 


1600 


2 


10 


H 


C 


2020 


27 


1500 


2 


10 


H 


C 


2061 


28 


1500 



2400 



0.02 



0.5 



0.5 



1.0 



0.75 



3/4 


6 


H 


7764 


5 


1500 


0.5 


3/4 


10 


H 


4460 


4 


1500 


0.5 


3/4 


10 


H 


7767 


4 


1500 


0.5 


3/4 


10 


H 


C70102E 


4 


1500 


0.5 


3/4 


12 


H 


C31005B 


6 


2000 


0.5 



0.75 



1-1/2 


10 


H 


C 


2067 


20 


1250 


0.75 


1-1/2 


10 


H 


C 


4438 


21 


1250 


0.75 


1-1/2 


10 


H 


C 


4439 


21 


1250 


0.75 


1-1/2 


10 


H 


C 


4440 


21 


1250 


0.75 


1-1/2 


10 


H 


C 


4441 


19 


1250 


0.75 


1-1/2 


10 


H 


C 


4441A 


19 


1250 


0.75 


1-1/2 


10 


H 


C 


4461 


19 


1500 


1.0 


1-1/2 


10 


H 


C 


6199 


18 


1250 


0.75 


1-1/2 


10 


H 


C 


C7151N 


22 


1600 


0.5 



0.5 



2.0 



2.0 



2 


10 


H 


C 


2062 


28 


1250 


0.75 


2 


10 


H 


V 


2063 


29 


2000 


2.0 


2 


10 


H 


C 


5819 


26 


1250 


0.75 


2 


10 


H 


C 


6342A 


30 


1500 


2.0 


2 


10 


H 


C 


6655A 


30 


1250 


0.75 


2 


10 


H 


1 


7746 


31 


2500 


2.0 


2 


10 


H 


V 


8053 


29 


2000 


2.0 


2 


12 


H 


1 


7850 


25 


2600 


2.0 


2 


14 


H 


1 


7264 


32 


2400 


2.0 



2.0 



Technical Data 



137 



TYPICAL CHARACTERISTICS AT THE 
SPECIFIED SUPPLY V0LTA6E AND 22° C 






Sensitivity 






Current 

Amplitication 

(Agproi.) 


Anode Dark 
Current nA@ 
Anode Luminous 
Sensitivity A/lm 








Radiant 


Luminous 


Spectral 


Voltaic 
V 


Cathode 
mA/W 


Anode 
A/W 


Cathodo 
uA/lm 


Anode 
A/lm 




1000 


44 


44,000 


35 


35 


1x10' 


20 ©20 




1000 


50 


120,000 


40 


100 


2.5 X 10 s 


5® 20 


104 (S-5) 


1000 


50 


250,000 


40 


200 


5x10' 


5 ©20 




1000 


2.3 


7500 


3 
40 


10 
100 


3.3 X 10' 
2.5 X 10 6 


6 ©0.8 
200 © 20 


105 (S-8) 


1000 


20 


50,000 


106 (S-10) 


1200 


48 


480 


60 


0.6 
7.5 


1x10* 
1.25 x 10' 


2 ©0.3 
6 ©7.5 




1250 


48 


6000 


60 




1250 


48 


12,800 


60 


16 

11 

155 


2.7 x 10' 
1.6 x 10' 
2.2 x 10" 


4 ©7.5 
1.4 ©7.5 
100 @ 100 




1250 


56 


8800 


70 




1500 


56 


120,000 


70 




1000 


36 


36,000 


45 


45 


lxio 6 


4.5 © 20 




1000 


60 


16,000 


74 


20 


2.7 X 10' 
6x10' 
6x10' 
6x10' 
6x10' 
6x10' 

1.7 X 10' 
1x10 s 


2.6 © 20 
16 ©20 
16 ©20 
16 ©20 
16 ©20 
16 ©20 
5 ©10 
4.5 © 20 




1000 


36 


22,000 


45 


27 




1000 


36 


22,000 


45 


27 
27 




1000 


36 


22,000 


45 




1000 


36 


22,000 


45 


27 
27 
10 
45 




1000 


36 


22,000 


45 




1250 


48 


8000 


60 


107 (S-ll) 


1000 


36 


36,000 


45 


1500 


70 


56,000 


85 


70 


8.1 X 10 5 


0.8 © 20 




1500 


70 


56,000 


85 


70 


8.1 X 10' 


0.8 © 20 




1250 


40 


4800 


50 


6 


1.2 X 10' 


4 ©20 




1250 


64 


— 


80 


- 


- 


— 




1000 
1500 


61 
56 


96,000 


76 
70 


120 


1.6 X 10 6 


6 ©20 




1000 


40 


80,000 


50 


100 


2x10' 


S@ 20 




1250 
1000 


64 
61 


25,000 
96,000 


80 
76 


31 
120 


3.9 X 10' 
1.6 X 10 6 


4 ©20 
6 ©20 




1500 


56 


100,000 


70 


130 


1.8 X 10 6 

6x10' 

9.1 X 10" 


16 ©20 

4@9 

64 © 160 




1500 


56 
56 
56 


34,000 


70 


42 




1800 


510,000 


70 


640 




2000 


3,400,000 


70 


4200 
3800 


6.1 X 10' 
5.4 X 10 7 


1000 © 2000 
1000 © 2000 




2000 


56 


3,000,000 


70 





138 



RCA Photomultiplier Manual 



PRELIMINARY SELECTION GUIDES 

For Photomultiplier Tubes by Spectral Response (cont'd) 



Spectral 



Nominal 
Tifct 

Diamtttr 
Inctiej 



Number Vinvint Cap 

ot Cmli|u- StriK- 

Staios ration 11 t»r» c 



MAXIMUM 
RATINGS 



RCA Outlnt, Suifly Averata 

Typo lasinf Voltat* Anodo 

No.* Dijf/im (E) Curront 

V mA 





3 


10 


H 


V 


2064B 


38 


2000 


2.0 


107 (S-ll) 


3 


10 


H 


V 


8054 


38 


2000 


2.0 


(Cont'd) 


5 


10 


H 


V 


2065 


41 


2000 


2.0 





5 


10 


H 


V 


8055 


41 


2000 


2.0 


108 (S-13) 


2 


10 


H 


c 


6903 


% 30 


1250 


0.75 


109 (S-19) 


1-1/8 


9 


S 


c 


7200 


14 


1250 


0.5 



110 (S-20) 



115 



3/4 



10 



8644 



3/4 



10 



8645 



1-1/2 



10 



C70114C 



19 



10 



4463 



29 



10 



7326 



33 



12 



4459 



25 



14 



7265 



32 



10 



4464 



38 



3/4 



10 



4516 



3/4 



10 



C70102M 



10 



C31016A 



10 



C31016B 



1-1/2 



10 



4517 



18 



TY/T 



10 



C7151Q 



22 



1-1/2 



10 



C70114F 



19 



1-1/2 



10 
10 



C70132A 

4518 

4523 



20 

30 

~29~ 



2100 



1800 



1800 



2500 



2400 



2800 



3000 



2500 



1800 



1800 



1500 



1500 



1800 



1800 



1800 



1800 
2000 
2500 



0.5 



0.1 



1.0 



1.0 



1.0 



1.0 



1.0 



1.0 





5 


10 


H 


V 


4465 


41 


2500 


1.0 


111 


1-1/2 


10 


DW 


C 


4526 


23 


2000 


0.1 


112 


1-1/2 


10 


H 


C 


C70114E 


19 


1800 


1.0 


3 


14 


S 


1 


C70045C 


39 


6000 


1.0 


113 


2 


12 


H 


1 


C31000A 


34 


3000 


1.0 


2 


12 


H 


1 


C31000B 


34 


3000 


1.0 




3/4 


10 


H 


1 


C70042D 


4 


2100 


0.5 


114 


2 


10 


H 


V 


C70109E 


29 


2500 


1.0 




2 


14 


H 


1 


C7268 


32 


3000 


1.0 



0.5 



0.5 



0.02 



0.02 



0.5 



0.5 



0.5 



0.5 
0.5 



Technical Data 139 



SUMlj 



TYPICAL CHARACTERISTICS AT THE 
SPECIFIED SUPPLY VOLTAGE AND 22° C 



SantRMty 



■ Dark 



RwHant Luminnut current Current nA@ Spectral 

VW Cathada Anada Cathada Anada *"£"*?" S^iT- fc "" se * 

V mA/W A/W uA/lm A/lm <*•»"»■> Sanstmty A/lm 



107 (S-ll) 
(Cont'd) 



1500 


64 


- 


80 


- 


— 


— 


1500 


64 


35,000 


80 


43 


5.4 x 10 5 


4@9 


1500 


88 


- 


110 


- 


- 


- 



1500 


88 


35,000 


110 


44 


4xl0 5 


4@9 




1000 


48 


72,000 


60 


90 


1.5 X 10 5 


10® 20 


168 (S-13) 


1000 


65 


65,000 


40 


40 


lxio 8 


4@20 


109 (S-19) 



1500 64 5100 150 12 8x10* 1.2® 30 

1500 64 5100 150 12 8x10* 1.2® 3ft 

1500 77 11,000 180 25 1.4 x 10 5 4 @ 10 

2000 68 11,000 160 25 1.6 x 10 5 4.8 @ 12 ' 

1800 64 38,000 150 88 5.9 x 10 5 3 @ 20 110 (S-20) 

2300 64 430,000 150 1000 6.6 x 10« 30 @ 300 

2400 64 3,000,000 150 7200 4 : 8 X 10 7 50 @ 1000 

2000 68 U,000 160 25 1.6 X 10 5 4.8 ©12 



2000 


68 


11,000 


-160 


25 


1.6 x 10 5 


4.8® 12 




1250 


89 


4400 


300 


15 


5xlO» 


2® 20 


111 


1500 


77 


11,000 


180 


25 


1.4 x 10 5 


4® 10 


112 


5000 


60 


- 


140 


- 


5xl0 6 


500 @ 1000 




2000 


77 


270,000 


200 


700 


3.5 x 10 6 


5® 200 


113. 


2000 


77 


270,000 


200 


700 


3.5 x 10" 


5® 200 


1500 


60 


4300 


140 


10 


7.1 x 10* 


6® 30 




1500 


68 


3400 


160 


8 


5x10* 


0.2® 6 


114 


2400 


64 


2,200,000 


150 


5200 


3.4 x 10' 


50 ©1000 





1500 71 56,000 60 47 8 x 10 5 0-2® 7 

1500 79 32,000 67 27 4 x 10 5 0.2 © 7 

1250 71 12,000 60 10 6 X 10 5 0.5 ©7 

1250 79 15,000 67 13 1.9 x 10 5 0.5 ® 7 

1500 79 56,000 67 47 7 X 10 5 0-2 ©7 115 

1500 79 39,000 67 33 5 x10 s 0-3 ©7 

1500 79 39,000 67 33 5 x10 s 0-3 ©7 

1500 79 65,000 67 55 8.2x10* 0.4 ©6.7 

1500 79 39,000 67 33 5 x 10 5 0.24® 7 

1500 71 32,000 60 27 4.5 X 10 5 0.5 ©13 



140 



RCA Photomultipiier Manual 



PRELIMINARY SELECTION GUIDES 

For Photomultipiier Tubes by Spectral Response (cont'd) 



Spectral 
Response* 



Nominal 

Tube 

Diameter 

Inches 



Number Viewing Cage RCA 

ol Configu- Struc- Type 

States ration 11 ture» No.* 



MAXIMUM 
RATINGS 



Outline, Supply Average 

Basing Voltage Anode 

Diagram (E) Current 

V mA 



115 

(Cont'd) 



3 


10 


H 


V 


4524 


38 


2500 


0.5 


5 


10 


H 


V 


4525 


41 


2500 


0.5 


5 


10 


H 


V 


C31027 


42 


2000 


0.5 



12 



C31029 



43 



2500 



0.5 





2 


12 


H 




8575 


34 


3000 


0.2 


116 


2 


12 


H 




8850 


34 


3000 


0.2 




2 


12 


H 




8851 


34 


3000 


0.2 


117 


1-1/2 


10 


H 


C 


C70114J 


19 


1800 


0.5 


3 


14 


S 




C70045D 


39 


6000 


1.0 


118 


5 


14 


H 




4522 


44 


3000 


0.5 


5 


14 


H 




C70133B 


44 


3000 


0.5 


119 


2 


12 


H 




C31000E 


34 


2500 


1.0 


2 


12 


H 




C31000F 


34 


2500 


1.0 


120 


2 


10 


H 


V 


8664 


35 


2000 


2.0 


2 


10 


H 


V 


8664/V1 


35 


2000 


2.0 


121 


3/4 


12 


H 


1 


C31005 


6 


2500 


0.5 


122 


3 


10 


H 


V 


4521 


40 


2000 


0.5 


123 


1/2 


9 


S 


C 


C70129E 


3 


1250 


0.02 


1-1/8 


9 


S 


C 


C31022 


15 


1250 


0.1 


124 


3/4 


10 


H 


1 


C70102N 


4 


1500 


0.5 


125 


3/4 


12 


H 


1 


C70128 


7 


1800 


0.5 


126 


2 


5 


H 


1 


C31024 


36 


6000 


0.5 


127 


1-1/2 


10 


H 


C 


C7151U 


24 


1250 


0.75 


128 


1-1/8 


9 


S 


C 


C31025C 


16 


1500 


0.1 


129 


1-1/8 


9 


S 


c 


C31025B 


16 


1800 


0.1 




1-1/8 


9 


S 


c 


C31025G 


16 


1800 


0.1 


130 


2 


10 


H 


c 


C7164R 


30 


1500 


0.5 


131 


3/4 


10 


H 


1 


C70042K 


4 


2100 


0.5 



Technical Data 



141 



TYPICAL CHARACTERISTICS AT THE 
SPECIFIED SUPPLY VOLTAGE AND 22° C 






Sensitivity 






Current 

Amplification 

(Approx.) 


Anode Dark 

Current nA@ 

Anode Luminous 

Sensitivity A/lm 






Radiant 


Luminous 


Spectral 


Voltage 

•V 


Cttmnle 
mA/VV 


tods 
A/W 


Cathode 
uA/lm 


Anode 
A/lm 


Response" 


1500 


71 
80 


32,000 


60 


27 


4.5 X 10 5 


1@13 




1500 


32,000 


67 


27 


4xl0 5 


1.5 @ 13 




1500 


88 


13,000 


77 


11.5 


1.5 X 10 5 


2 @0.9 
A/incident lm d 


115 
(Cont'd) 


1750 


88 


130,000 


77 


115 


1.5 X 10 6 


20® 9 
A/incident lm d 




2000 


97 


970,000 


85 


850 


1x10' 


1@200 




2000 


97 
97 


710,000 


85 


620 


7.3 X 10 6 


0.6 @ 200 


116 


2000 


710,000 


85 


620 


7.3 x 10 s 


0.6 @ 200 




2500 


79 
72 
88 
88 


39,000 


67 


33 


5x 10 5 


0.3® 7 


117 


5000 


— 


- 


- 


1x10' 


1000 @ 1000 




2000 


2,600,000 


77 


2300 


3x10' 


60 @ 2000 


118 


2000 


2,600,000 


77 


2300 


3x10' 


60 @ 2000 




1500 


45 


18,000 


250 


100 


4 x10 s 


10® 30 
10® 30 


119 


1500 


45 
69 


18,000 


250 


100 


4xl0 5 




1500 


18,000 


67 


17 


2.6 x 10 5 


1 @7.5 


120 


1500 


69 

9.2 min.® 
253.7 nm 


18,000 


67 


17 


2.6 x 10 5 


1@7.5 




2100 


9200 min.® 
253.7 nm 


- 


- 


lxlO 6 


0.1 @ 3000 
A/W 


121 


1500 


87 


19,000 


83 


18 


2.2 x 10 5 


2® 7.5 


122 


1000 


31 


21,000 


30 


20 


6.7 X 10 5 


8® 15 


123 


1000 


48 
72 


160,000 
4800 


60 


200 


3.3 x 10 6 


2® 40 




1250 


90 


6 


6.7 x 10« 


2® 7.5 


124 


1500 


10 @ 
253.7 nm 


3000® 
253.7 nm 


- 


- 


3xl0 5 


0.1 @ 3000 
A/W 


125 


3000 


82 


110,000 


72 


93 


1.3 X 10 6 


24® 85 


126 


1000 


25 


8200 


60 


20 


3.3 x 10 5 


10 ©40 


127 


1250 


33 
45 


1300 


250 


10 


4xl0« 


0.5 @ 10 


128 


1250 


2000 


160 


7 


4.4 X 10« 


0.5® 10 


129 


1250 


28 


1400 


100 


5 


5x10* 


0.3® 5 




1250 


40 


20,000 


200 


100 


5xl0 5 


30 @ 150 


130 


1500 


45 


3600 


250 


20 


8X10 4 


6® 30 


131 



142 



RCA Photomultiplier Manual 



PRELIMINARY SELECTION GUIDES 

For Photomultiplier Tubes by Spectral Response (cont'd) 



Nominal 
Spectral Tube 

Response* Diameter 

Inches 



Number Viewing Cage RCA 

of Configu- Struc- Type 

States ration b tore" No.* 



MAXIMUM 
RATIN6S 



Outline, Supply Average 
Basing Voltage Anode 

Diajriin (E) Current 
V mA 



132 



3/4 


10 


H 


C70042R 


4 


2100 


0.5 


1 


12 


H 


C31026 


10 


2200 


0.5 



1-1/2 



10 



C7151W 



18 



1500 



0.5 



0.5 



133 



136 



137 



138 



3/4 



10 



C70042J 



1-1/8 



C31028 



11 



C31000K 



1-1/2 



10 



C7151Y 



17 



37 



24 



1800 



1250 



2000 



1500 



0.5 



0.1 



0.5 



* See bar chart of Spectral response on page 156. 

* Viewing configurations: H, head on; S, side on; 
and DW, dormer window. 

c Cage structures: C, circular cage; I, in line; and 

V, Venetian blind. 
a With blue light source. Tungsten light source at 

color temperature of 287D*K. Corning C.S. No. 5-58, 

1/2 stock thickness. 



• Type numbers with prefix C are developmental 
types. Each of these C numbers identifies a par- 
ticular laboratory tube design but the number and 
the identifying data are subject to change. No 
obligations are assumed as to future manufacture 
unless otherwise arranged. 



PRELIMINARY SELECTION GUIDES 
For Photomultiplier Tubes by Diameter 



Nominal 

Tube Spectral 

Diameter Response* 

Inches 



Number Viewini Cage RCA 

of Configu- Struc- Type 

States ration 11 tore No.* 



MAXIMUM 
RATINGS 



Outline, Supply Average 

Basing Voltage Anode 

Diagram (E) Current 



1/2 



3/4 



102 (S-4) 


9 


S 


C 


8571 


1 


1250 


0.02 


104 (S-5) 


9 


s 


C 


U/0129H 


2 


1250 


0.02 


123 


9 


s 


C 


C70129G 


3 


1250 
1500 


0.02 


101 (S-1) 


10 


H 


1 


C70102B 


4 


0.01 



107 (S-1 1) 



12 



C31005B 



6 


H 


7764 


5 


1500 


0.5 


10 


H 


4460 


4 


1500 


0.5 


10 


H 


7767 


4 


1500 


0.5 


10 


H 


C70102E 


4 


1500 


0.5 



2000 



0.5 



Technical Data 143 



¥ 



TYPICAL CHARACTERISTICS AT THE 
SPECIFIED SUPPLY VOLTAGE AND 22° C 



Sensitivity 



Radiant Luminous 



Supply 

Voriaie cathode Anode Cathode 



bA/W A/W uA/lm A/lm 





Anode Dark 




Current 


Current nA@ 


Spectral 


AmpMcalion 


Anode Luminous 


Response 


(Appro*.) 


Sensitivity A/lm 





1500 


44 


5500 


200 


25 


1.25 x 10 5 


2@30 




1800 


43 


26,000 


250 


150 


6x105 


40® 50 


132 


1250 


40 


10,000 


200 


50 


2.5 x 10 5 


1@20 




1800 


71 


56,000 


60 


47 


8xl0 5 


0.2® 7 


133 


1000 


54 


175,000 


65 


200 


3.1 x 10« 


0.8 @ 20 


136 


1500 


35 


21,000 


330 


200 


6 x10 s 


6@100 


137 


1250 


40 


2400 


200 


12 


6x10* 


3® 10 


138 



TYPICAL CHARACTERISTICS AT THE 
SPECIFIED SUPPLY V0LTA8E AND 22° C 



Sensitivity 



Anode Dart 



Supply H***" 1 UrmlMue Current Current oA@ Tilt 

WJ* Cathode Anode Cathode Anode *JJ*™S , «" i^","!!'? """" 

V mA/W A/W uA/lm A/lm (*"»•«) SentlthHy A/lm 



1000 


34 


73,000 


35 


75 


2.1 x 10« 


2® 20 




1000 


44 


44,000 


35 


35 


ixio« 


20® 20 


1/2 


1000 


31 


21,000 


30 


20 


6.7 x 10 s 


8@15 





1250 2.8 310 30 3.3 1.1 x 10 s 800® 4 

1200 48 480 60 0.6 1x10* 2® 0.3 

1250 48 6000 60 7.5 1.25 x 10 5 6 ©7.5 3/4 

1250 48 12,800 60 16 2.7 x 10 5 4 ® 7.5 

1250 56 8800 70 11 1.6 X 10 5 1.4® 7.5 

1500 56 120,000 70 155 2.2 x IP 6 100® 100 



144 



RCA Photomultiplier Manual 



PRELIMINARY SELECTION GUIDES 

For Photomultiplier Tubes by Diameter (cont'd) 



Nominal 


Spectral 
Response" 


Number 

of 
States 


Viewing 
Configu- 
ratton b 


Cage 
Struc- 
ture 


RCA 
Type 
No.* 


Outline, 
Basing 
Diagram 


MAXIMUM 
RATINGS 


Tube 
Diameter 
Inches 


Supply 

Voltage 

(E) 

V 


Average 

Anode 

Current 

mA 




110 (S-20) 


10 


H 




8644 


4 


2100 
1800 


0.5 




10 


H 




8645 


8 


0.1 




114 


10 


H 




C70042D 


4 


2100 


0.5 




115 


10 


H 




4516 


4 


1800 


0.5 




10 


H 




C70102M 


4 


1800 


0.5 


3/4 
(Cont'd) 


121 


12 


H 




C31005 


6 


2500 


0.5 




124 


10 


H 




C70102N 


4 
7 


1500 
1800 


0.5 




125 


12 


H 




C70128 


0.5 




131 


10 


H 




C70042K 


4 
4 


2100 
2100 


0.5 




132 


10 


H 




C70042R 


0.5 




133 


10 


H 




C70042J 


4 


1800 


0.5 




115 


10 


H 


C 


C31016A 


9 


1500 
1500 
2200 


0.02 


1 


10 


H 


C 


C31016B 


9 
10 


0.02 




132 


12 


H 


1 


C31026 


0.5 




101 (S-l) 


9 


S 


C 


C31004A 


11 


1500 


0.01 




102 (S-4) 


9 


S 


C 


1P21 


11 


1250 
1250 
1250 


0.1 




9 


S 


C 


931A 


11 


1.0 




9 


S 


C 


4471 


11 


1.0 




9 


S 


C 


4472 


11 


1250 


1.0 




9 


S 


C 


4473 


11 


1250 


0.1 




9 


S 


C 


6328 


12 


1250 
1250 


0.1 




9 


S 


C 


6472 


13 


0.1 


1-1/8 


9 


S 


C 


7117 


12 
11 


1250 
1250 


0.1 




9 


S 


C 


C7075J 


0.1 




103 


9 


S 


C 


1P28/V1 


11 


1250 


0.5 




9 


S 


C 


1P28A/V1 


11 
11 


1250 
1250 


0.5 




104 (S-5) - 


9 


S 


C 


1P28 


0.5 




9 


S 


C 


1P28A 


11 


1250 


0.5 




105 (S-8) 


9 


S 


C 


1P22 


11 


1250 


1.0 




109 (S-19) 


9 


S 


C 


7200 


14 
15 


1250 
1250 


0.5 




123 


9 


S 


c 


C31022 


0.1 



Technical Data 



145 



Suppry 

Vallate 

V 



1500 



1500 



1500 



1500 



1500 



1250 



1500 



1500 



1500 



1800 



1250 



1250 



1800 



1250 



1000 



1000 



1000 



TYPICAL CHARACTERISTICS AT THE 
SPECIFIED SUPPLY VOLTAGE AND 22° C 



Sensitivity 



Radiant 



Cathode 
mA/W 



Anode 
A/W 



Cathode 
uA/lm 



Anode 
A/lm 



Anode Dark Nominal 

Current Current nA@ Tube 

Amplification Anode Luminous Diameter 

(Approx.) Sensitivity A/lm Inches 



64 



5100 



64 



5100 



60 



4300 



71 



56,000 



79 



32,000 



1000 



1000 



1000 



1000 



1000 



1000 



1000 



1000 
1000 
1000 
1000 
1000 
1000 



72 



10® 
253.7 nm 



45 



44 



71 



71 



79 



43 



1.9 



40 



40 



40 



40 



40 



40 



48 



48 
50 
50 
2.3 
65 
48 



4800 



3000® 
253.7 nm 



3600 



5500 



56,000 



12,000 



15,000 



26,000 



235 



120,000 



83,000 



100,000 



100,000 



160,000 



35,000 



32,500 



35,000 



83,000 



160,000 



160,000 
120,000 
250,000 
7500 
65,000 
160,000 



150 



150 



140 



60 



67 



2100 9.2 min.® 9200 min.® - 
253.7 nm 253.7 nm 



90 



250 



200 



60 



60 



67 



250 



20 



40 



40 



40 



40 



40 



40 



60 



60 
40 
40 



40 
~6b~ 



12 



8x10* 



3® 30 



12 



8x10* 



3® 30 



10 



7.1 x 10* 



6® 30 



47 



8 x 10 5 0.2® 7 



27 



4 x 10 5 0.2 @ 7 



lxio 6 

min. 



0.1 @ 3000 
A/W 



6.7 x 10* 



2® 7.5 



3xl0 5 



0.1 @ 3000 
A/W 



20 



8x 10* 



6® 30 



25 1.25 x 10 5 



2® 30 



47 



8 X 10 5 0.2 @ 7 



10 



6 X 10 5 0.5 @ 7 



13 



1.9 x 10 5 0.5 @ 7 



150 



6 x10 s 



40® 50 



2.5 1.25 x 10 5 300 @ 2 



120 



3x 10 6 



80 



2x 10 6 



1@20 
5® 20 



100 



2.5 x 10 6 



5® 20 



100 



2.5 x 10 6 



5® 20 



160 



4x 10 6 



1@20 



35 



35 



35 



80 



2x 10 6 



5 ©.20 



200 



3.3 x 10 6 



2® 40 



200 
100 
200 
10 
40 
200 



3.3 X 10 6 
2.5 x 10" 

5xl0 6 
3.3 x 10" 

lxlO 6 
3.3 x 10« 



2® 40 
5® 20 
5 ©20 
6® 0.8 
4 ©20 
2 ©40 



3/4 
(Cont'd) 



1-1/8 



146 RCA Photomultiplier Manual 

PRELIMINARY SELECTION GUIDES 

For Photomultiplier Tubes by Diameter (cont'd) 



1-1/8 
(Cont'd) 



1-1/2 



MAXIMUM 
RATINGS 



Tlbi Spactril Nurabtr Vinrini Cap RCA OulliM, SomIi Avtrm 

Sti|« nBonb tura° No.* Hapm (E) Currtat 



V nA 



128 9 S C C31025C 16 1500 0.1 



129 


9 


S 


C 


C31025B 


16 


1800 


0.1 




9 


S 


C 


C31025G 


16 


1800 
1250 
1500 


0.1 


136 


9 


S 


C 


C31028 


17 
18 


0.5 


101 (S-l) - 


10 


H 


C 


7102 


0.01 




10 


H 


C 


C70114D 


19 
18 


1500 
1250 


0.01 




10 


H 


C 


2060 


0.75 




10 


H 


C 


2067 


20 


1250 


0.75 




10 


H 


C 


4438 


21 


1250 


0.75 




10 


H 


C 


4439 


21 


1250 


0.75 




10 


H 


C 


4440 


21 


1250 


0.75 


107 (S-ll) 


10 


H 


C 


4441 


19 


1250 


0.75 




10 


H 


C 


4441A 


19 


1250 


0.75 




10 


H 


C 


4461 


19 


1500 


1.0 




10 


H 


C 


6199 


18 


1250 


0.75 




10 


H 


C 


C7151N 


22 


1600 


0.5 



10 H C C70132B 20 1600 0.5 



110 (S-20) 10 H C C70I14C 19 1800 1.0 



111 10 DW C 4526 23 2000 0.1 



H2 10 H C C70114E 19 1800 1.0 

115 

~~ 10 H C C70132A 20 1800 oT 



10 


H 


C 


4517 


18 


1800 


0.5 


10 


H 


C 


C70114F 


19 


1800 


0.5 


10 


H 


C 


C7151Q 


22 


1800 


0.5 



117 10 H C C70114J 19 1800 0.5 



127 10 H C C7151U 24 1250 0.75 



132 10 H C C7151W 18 1500 0.5 



138 10 H C C7151Y 24 1500 0.5 



101 (S-l) 12 H I C70007A 25 2000 0.01 



106 (S-10) 10 H C 6217 26 1250 0.75 



2 10 H C 2020 27 1500 2.0 



107 (S-ll) 10 H C 2061 28 1500 2.0 
10 H C 2062 28 1250 otT 



Technical Data 147 



TYPICAL CHARACTERISTICS AT THE 
SPECIFIED SUPPLY VOLTAGE AND 22° C 






SentithitJ 






Currant 

Amplication 

(Apart*-) 


Anode Dark 
Current nA@ 
Anode Luminous 
SenHMtyA/lm 




S«P»lj . 
Voltaii 
V 


Cathode 
mA/W 


Radiant 

Anode 
A/W 


Uminooi 

Cithodi Anada 
uA/lm A/lin 


Nominal 
Ttte 

Diameter 
iKhtt 


1250 


33, 


1300 


250 


10 


4X10 4 


0.5 @ 10 




1250 


45 


2000 


160 


7 


4.4 X 10* 


0.5 @ 10 


1-1/8 


1250 


28 


1400 


100 


5 


sxio* 


0.3® 5 


(Cont'd) 


1000 


54 


175,000 


65 


200 


3.1 x 10 s 


0.8 ©20 




1250 


2.8 


660 


30 


7 


2.3 x 10 5 


1900@4 




1250 


2.8 


420 


30 


4.5 


1.5 X 10 5 


1700® 4 




1000 


36 


36,000 


45 


45 


1x10 s 


4.5® 20 




1000 


60 


16,000 


74 


20 


2.7 x 10 5 


2.6® 20 




1000 


36 


22,000 


45 


27 


6xl0 5 


16 @ 20 




1000 


36 


22,000 


45 


27 


6xl0 5 


16® 20 




1000 


36 


22,000 


45 


27 


6 x10 s 


16® 20 




1000 


36 


22,000 


45 


27 


6 x10 s 


16® 20 




1000 


36 


22,000 


45 


27 


6 x10 s 


16® 20 




1250 


48 


8000 


60 


10 


1.7 x 10 s 


5® 10 




1000 


36 


36,000 


45 


45 


ixio« 


4.5® 20 


1-1/2 


1500 


70 


56,000 


85 


70 


8.1 x 10 s 


0.8 @ 20 




1500 


70 


56,000 


85 


70 


8.1 x 10 s 


0.8 @ 20 




1500 


77 


11,000 


180 


25 


1.4 x 10 s 


4® 10 




1250 


89 


4400 


300 


15 


SxlO 4 


2@20 




1500 


77 


11,000 


180 


25 


1.4 x 10 s 


4® 10 




1500 


79 


56,000 


67 


47 


7 x10 s 


0.2® 7 




1500 


79 


39,000 


67 


33 


5 x10 s 


0.3® 7 




1500 


79 


39,000 


67 


33 


5 x10 s 


0.3® 7 




1500 


79 


65,000 


67 


55 


8.2 x 10 s 


0.4 @ 6.7 




1500 


79 


39,003 


67 


33 


5 x10 s 


0.3® 7 




1000 


24 


8200 


60 


20 


3.3 x 10 s 


10@40 




1250 


40 


10,000 


200 


50 


2.5 x 10 s 


1@20 




1250 


40 


2400 


200 


12 


6x10* 


3® 10 




1250 


2.8 


940 


30 


10 


3.3 x 10 s 


400@4 




1000 


20 


50,000 


40 


100 


2.5 X 10" 


200® 20 




1250 


40 


4800 


50 


6 


1.2 x 10 s 


4® 20 


2 


1250 


64 


— 


80 


— 


— 


— 




1000 


61 


96,000 


76 


120 


1.6 X 10 6 


6@20 





148 



RCA Photomultiplier Manual 



PRELIMINARY SELECTION GUIDES 

For Photomultiplier Tubes by Diameter (cont'd) 



Nominal 

Tuba Spectral 

Diameter Response* 

Inches 



Number 

of 
States 



Viewing Cage 
Configu- Struc- 
ration b ture« 



HC* 
Type 
No.* 



MAXIMUM 
RATINGS 



Outline, Supply Average 

Basing Voltage Anode 

Diagram (£) Current 

V mA 



10 



2063 



29 



10 



5819- 



26 



10 



6342A 



JU 



I07(S-il) 
(Cont'd) 



10 



6655A 



30 



10 



7746 



31 



10 



8053 



29 



12 



7850 



25 



14 



6810A 



32 



14 



7264 



32 



108 (S-13) 10 



6903 



30 



10 



4463 



29 



110 (S-20) 



10 



7326 



33 



12 



4459 



25 



2000 



1250 



1500 



1250 



2500 



2000 



2600 



2400 



2400 



1250 



2500 



2400 



2800 



O./S 



2.0 



0.75 



2.0 



2.0 



2.0 



2.0 



2.0 



0.75 



1.0 



1.0 



1.0 





14 


H 


1 


7265 


32 


3000 


1.0 


2 
(Cont'd) 113 


12 


H 


1 


C31000A 


34 


3000 


1.0 


12 


H 


1 


C31000B 


34 


3000 


1.0 


114 


10 


H 


V 


C70109E 


29 


2500 


1.0 


14 


H 


1 


C7268 


32 


3000 


1.0 


115 


10 


H 


C 


4518 


30 


2000 


0.5 


10 


H 


V 


4523 


29 


2500 


0.5 




12 


H 


i 


8575 


34 


3000 


0.2 


116 


12 


H 


1 


8850 


34 


3000 


0.2 




12 


H 


1 


8851 


34 


3000 


0.2 


119 


12 


H 


1 


C31000E 


34 


2500 


1.0 


12 


H 


1 


C31000F 


34 


2500 


1.0 


120 


10 


H 


V 


8664 


35 


2000 


2.0 


10 


H 


V 


8664/V1 


35 


2000 


2.0 


126 


5 


H 


1 


C31024 


36 


6000 


0.5 


130 


10 


H 


C 


C7164R 


30 


1500 


0.5 


137 


11 


H 


1 


C31000K 


37 


2000 


0.1 


3 107 (S-11) - 


10 


H 


V 


2064B 


38 


2000 


2.0 


10 


H 


V 


8054 


38 


2000 


2.0 


110 (S-20) 


10 


H 


V 


4464 


38 


2500 


1.0 



Technical Data 149 



TYPICAL CHARACTERISTICS AT THE 
SPECIFIED SUPPLY VOLTAGE AND 22° C 






Sensitivity 






Current 
Amplification 

(Aaprox.) 


Anode Dark Nominal 
Current nA@ Tube 






Radiant 


Luminous 


Voltage 

V 


Cathode 
mA/W 


Anode 
A/W 


Cathode 
uA/lm 


Anode 
A/lm 


Anode Luminous Diameter 
Sensitivity A/lm Inches 


1500 


56 
40 
64 


_ 


70 


— 


— 


— 


1000 


80,000 


50 


100 


2X10 6 


6@20 


1250 


25,000 


80 


31 


3.9 x 10 5 


4@20 


1000 


61 
56 
56 
56 
56 
56 


96,000 


76 


120 


1.6 x 10 6 


6@20 


1500 


100,000 


70 


130 


1.8 x 10 6 


16® 20 


1500 


34,000 
510,000 


70 
70 


42 
640 


6xl0 5 
9.1 x 10 6 


4@9 


1800 


64 @ 160 


2000 


3,000,000 


70 


3800 


5.4 x 10 7 


1000 @ 2000 


2000 


3,400,000 


70 


4200 


6.1 x 10 7 


1000 @ 2000 


1000 


48 
68 
64 


72,000 


60 


90 


1.5 x 10 6 


10® 20 


2000 


11,000 


160 


25 


1.6 x 10 5 


4.8® 12 


1800 


38,000 


150 


88 


5.9 x 10 5 


3@20 


2300 


64 
64 
77 
77 
68 
64 
79 
71 
97 


430,000 


150 


1000 


6.6 X 10 6 


30 @ 300 


2400 


3,000,000 


150 


7200 


4.8 x 10 7 


50 @ 1000 


2000 


270,000 


200 


700 


3.5 x 10 6 


5 ©200 2 


2000 


270,000 


200 


700 


3.5 x 10 6 


5 @ 200 (Cont'd) 


1500 


3400 


160 


8 


5x10* 


0.2® 6 


2400 


2,200,000 


150 


5200 


3.4 x 10 7 


50 @ 1000 


1500 


39,000 


67 


33 


5x 10 5 


0.24 @ 7 


1500 


32,000 


60 


27 


4.5 x 10 5 


0.5 @ 13 


2000 


970,000 


85 


850 


lxlO 7 


1 ©200 


2000 


97 
97 

45 
45 


710,000 


85 


620 


7.3 X 10 6 


0.6 @ 200 


2000 


710,000 


85 


620 


7.3 X 10 6 


0.6 @ 200 


1500 


18,000 


250 


100 


4xl0 5 


10® 30 


1500 


18,000 


250 


100 


4xl0 5 


10® 30 


1500 


69 


18,000 


67 


17 


2.6 X 10 5 


1@7.5 


1500 


69 
82 


18,000 


67 


17 


2.6 X 10 5 


1@7.5 


3000 


110,000 


72 


93 


1.3 x 10 6 


24® 85 


1250 


40 


20,000 


200 


100 


5xl0 5 


30 @ 150 


1500 


35 
64 
64 


21,000 


330 


200 


6xl0 5 


6 ©100 


1500 


— 


80 


— 


- 


- 


1500 


35,000 


80 


,*3 


5.4 x 10 5 


4®9 3 


2000 


68 


11,000 


160 


25 


1.6 X 10 s 


4.8 @ 12 



150 



RCA Photomultiplier Manual 



PRELIMINARY SELECTION GUIDES 

For Photomultiplier Tubes by Diameter (cont'd) 



Nominal 

Tube 

Diameter 

Indies 



Spectral 



MAXIMUM 
RATINGS 



Number 


Viewini 


Caie 


RCA 


Outline, 


Supply 


Average 


ot 


Confifii- 


Struc- 


Type 


Bating 


Voltaie 


Anode 


States 


rafion b 


ture 1 


No.* 


Diatram 


(E) 
¥ 


Current 
mA 



112 


14 


S 


1 


C70045C 


39 


6000 


1.0 


3 115 


10 


H 


V 


4524 


38 


2500 


0.5 


(Cont'd) 117 


14 


S 


1 


C70045D 


39 


6000 


1.0 


122 


10 


H 


V 


4521 


40 


2000 


0.5 


107 (S-11) 


10 


H 


V 


2065 


41 


2000 


2.0 


10 


H 


V 


8055 


41 


2000 


2.0 


110 (S-20) 


10 


H 


V 


4465 


41 


2500 


1.0 



115 



10 



10 



4525 



C31027 



41 



42 



2500 



2000 



0.5 



0.5 



12 



C31029 



43 



2500 



0.5 



118 



14 



14 



H 



4522 



44 



C70133B 



44 



3000 



3000 



0.5 



0.5 



• See bar chart of Spectral response on page 156. 
b Viewing configurations: H, head on; S, side on; 

and DW, dormer window. 
■ Cage structures: C, circular cage; I, in line; and 
V, Venetian blind. 

* With blue light source. Tungsten light source at 
color temperature of 2870°K. Corning C.S. No. 5-58, 
1/2 stock thickness. 



• Type numbers with prefix C are developmental 
types. Each of these C numbers identifies a par- 
ticular laboratory tube design but the number and 
the identifying data are subject to change. No 
obligations are assumed as to future manufacture 
unless otherwise arranged. 



PRELIMINARY SELECTION GUIDES 
Ruggedized Photomultiplier Tubes by Size 



Tube 



Spectral 
Response* 



Viewini 
Confieu- 
ratfon b 



Caie 
Structure' 



RCA 
Type 



Military _,. 
Specification" 



1/2 



S-4 
(102) 



S-5 
(104) 



8571 



C70129H 



3/4 



S-l 
(101) 



C70102B 



MIL-E-5272C 



Technical Data 



151 



TYPICAL CHARACTERISTICS AT THE 
SPECIFIED SUPPLY VOLTAGE AND 22° C 



SMSitMtjM 



Supply 

Voltaic cuwi,, 

v mA/W 



Radiant 



Luminous 



A/W 



Catheds 
uA/lm 



Anode 
A/lm 



Currant 

AmpUticatiofl 

(Approx.) 



Anodo Dark 
Currant nA@ 
Anode Luminous 
SensitMti A/lm 



Nominal 
Take 

Diametor 
laches 



5000 



1500 



5000 



1500 



60 



71 



72 



87 



HO 



5 x 10* 500 @ 1000 



32,000 



60 



27 4.5 X 10 5 



1@13 



1 x 10 7 1000 @ 10,000 (Cont'd) 



19,000 



83 



18 



2.2 x 10 5 



2® 7.5 



1500 



1500 



2000 



1500 



68 



80 



1750 



2000 



2000 



110 



35,000 



110 



44 



4xl0 5 



4@9 



11,000 



160 



25 



1.6 x 10 s 4.8 @ 12 



32,000 



67 



27 



4 x 10 5 1-5 @ 13 



1500 88 13,000 77 11.5 1.5 X 10 5 2 @ 0.9 

A/incident lm d 



130,000 



77 



11.5 1.5 xlO 6 20® 9 

A/incident lm d 



2,600,000 



77 2300 3xl0 7 60 ©2000 



88 2,600,000 



77 2300 



3x10' 



60 @ 2000 



Quality 
Conformance 
Inspection' 



Shock 



Design 



100% 
Design 



30 g 
11 ms 



30 g 
11 ms 

30 g 
11 ms 



ENVIRONMENTAL TESTING 6 



Vibrations 



Acceleration 



Temperature 
Cydini 



Design 30 g 20 g 
11 ms 5-2000 Hz 1-1/2 hrs 



15 g 
5min 



-45 +75°C 
8 hrs 



20 g 

5-2000 Hz 1-1/2 hrs 



15 g 
5min 



-45+75°C 
8 hrs 



20 g 

20-2000 Hz 15 min 

20 g 

20-2000 Hz 6 hrs 



100 g 
1 min 



Tube 

Dia mater 

Inches 



1/2 



3/4 



152 RCA Photomultiplier Manual 



PRELIMINARY SELECTION GUIDES 

Ruggedized Photomultiplier Tubes by Size (cont'd) 



Nominal 
Tuln 

DIametar 
Indies 


Spectral 
Response" 


Viewini 
Confiju- 
ralion b 


Cage 
Structure 


RCA 

Type 

Number 


Military 
Specillcation d 




S-ll 
(107) 


H 


1 


4460 


MIL-E-5272C 


3/4 


S-ll 
(107) 


H 


1 


C70102E 


MIL-E-5272C 


(Cont'd) 


115 


H 


1 


C70102M 


MIL-E-5272C 




124 


H 


1 


C70102N 


MIL-E-5272C 


1 


115 
115 


H 
H 


C 
C 


C31016A 
C31016B 


MIL-STD 
810 B 

MIL-STD 
810 B 




S-ll 
(107) 


H 


C 


4441A 


MIL-E-5272C 




S-ll 
(107) 


H 


C 


4461 


MIL-E-5272C 


1-1/2 


S-ll 
(107) 


H 


c 


C7151N 


MIL-E-5272C 




115 


H 


c 


C7151Q 


MIL-E-5272C 




S-20 
(110) 


H 


c 


C70114C 


MIL-E-5272C 




S-l 
(101) 


H 


c 


C70114D 


MIL-E-5272C 



Technical Data 



153 







ENVIRONMENTAL TESTING" 




Nominal 


Quality 
Contormanct 
Inspection' 


shock 


Vibrations 


tiim 

Acceleration Temperature PS2!? r 
Cyclint lncllM 


100% 

Design 


30 g 
11 ms 


20 g 

20-2000 Hz 15 min 

20 g 

20-2000 Hz 6 hrs 


100 g 
1 min 




100% 
Design 


30 g 
11 ms 

30 g 
11 ms 


20 g 

20-2000 Hz 15 min 

20 g 

20-2000 Hz 6 hrs 


100 g 
1 min 


3/4 


100% 
Design 


30 g 
11 ms 

30 g 
11 ms 


20 g 

20-2000 Hz 15 min 

20 g 

20-2000 Hz 6 hrs 


100 g 
1 min 


(Cont'd) 


100% 
Design 


30 g 
11 ms 

30 g 
11 ms 


20 g 

20-2000 Hz 15 min 

20 g 

20-2000 Hz 6 hrs 


100 g 
1 min 




Design 
Design 


75 g 
11 ms 

75 g 
11 ms 


20.7 g (rms) 
50-2000 Hz 1-1/2 hrs 

20.7 g (rms) 
50-2000 Hz 1-1/2 hrs 


100 g 
100 g 


1 


100% 
Design 


30 g 
11 ms 


20 g 

20-2000 Hz 15 min 

20 g 

20-2000 Hz 6 hrs 


100 g 
1 min 




100% 
Design 


30 g 
11 ms 


20 g 

20-2000 Hz 15 mm 

20 g 

20-2000 Hz 6 hrs 


100 g 
1 min 




100% 
Design 


30 g 
11 ms 


20 g 

20-2000 Hz 15 min 

20 g 

20-2000 Hz 6 hrs 


100 g 
1 min 


1-1/2 


Design 


30 g 
11 ms 


20 g 

20-2000 Hz 6 hrs 


100 g 
1 min 




Design 


30 g 
11 ms 


20 g 

20-2000 Hz 6 hrs 


100 g 
1 min 




100% 
Design 


30 g 
11 ms 

30 g 
11 ms 


20 g 

20-2000 Hz 15 min 

20 g 

20-2000 Hz 6 hrs 


100 g 
1 min 





154 



RCA Photomultiplier Manual 



PRELIMINARY SELECTION GUIDES 

Ruggedized Photomultiplier Tubes by Size (cont'd) 



Tube Spectral 

Diameter RetpmsB* 

Indies 



Viawinj 

ConfifU- Ct(e 

ra«en b Structurt c 



RCA 

Type 

Number 



Military 

Specification' 1 



112 



C70114E 



MIL-E-5272C 



1-1/2 
(Cont'd) 



115 


H 


C 


C70114F 


MIL-E-5272C 


115 


H 


C 


C70132A 


MIL-E-5272C 


117 


H 


C 


C70114J 


MIL-E-5272C 



S-ll 
(107) 



C70132B 



MIL-E-5272C 



120 



8664 



120 



8664/V1 



■ See bar chart of spectral response on page 156. 
b Viewing configurations: H, head on; and S, side on. 
c Cage structures: C, circular cage; I, in line; and 

V, Venetian biind. 
4 Military Specifications: 

MIL-E-5272C, 13 April 1959, Amendment 1, 5 

January 1960; 
MIL-STD-810B, 15 June 1967 



• For detailed information on environmental testing, 
request a technical data sheet on the specific type. 

' Quality Conformance Inspection: 100%, every tube 
tested; sample, some tubes tested from each lot; 
and Design, initial tubes only are tested. 

s Vibration: Cycling ranges from minimum; to maxi- 
mum to minimum; time is total time for vibration 
in three axes (equal time for each axis). 



Technical Data 



155 







ENVIRONMENTAL TESTING 1 * 






Nominal 


Quality 
Conlormance 
Inspection' 


shock 


Vibration^ 


Acceleration 


Temperature 
Cydinf 


Tuba 

Diameter 

indies 


Design 


30 g 
11 ms 


20 g • 
20-2000 Hz 6 hrs 


100 g 
1 min 






Design 


30 g 
11 ms 


20 g 

20-2000 Hz 6 hrs 


100 g 
1 min 






Design 


30 g 
11 ms 


20 g 

20-2000 Hz 6 hrs 


100 g 
1 min 




1-1/2 
(Cont'd) 


Design 


30 g 
11 ms 


20 g 

20-2000 Hz 6 hrs 


100 g 
1 min 






100% 
Design 


30 g 
11 ms 


20 g 

20-2000 Hz 15 min 

20 g 

20-2000 Hz 6 hrs 


100 g 
1 min 






Sample 
Design 


150 g 
11 ms 
1500 g 
50 ms 


60 g 

48-3000 Hz 15 min 

60 g 

48-3000 Hz 6 hr 


150 g 
6 min 




2 


Sample 
Design 


150 g 
11 ms 
1500 g 
50 ms 


60 g 

48-3000 Hz 15 min 

60 g 

48-3000 Hz 6 hr 


150 g 
6 hr 







156 



RCA Photomultiplier Manual 



SPECTRAL RESPONSE AND 

RELATED PHOTOCATHODE CHARACTERISTICS 



SPECTRAL 
RESPONSE 

RCA EIA Cathode 
Code Desii- Material 
No. nation 



Window 
Material 



USEFUL SPECTRAL RAN6E 

■■■■■.■» ♦»....■ i...... ^.■.'.■.■'■.'.■■■a-.i] 



TYPICAL DATA 



101 S-l Ag-O-Cs L« 



102 S-4 Cs-Sb 



Lime or 
Borosil. 



103 



Cs-Sb UV Glass 



104 S-5 Cs-Sb UV Glass 



105 S-8 Cs-Bi 



Lime or 
Borosil. 



106 S-in A &" Bi " Lime or 
lub blu O-Cs Borosil. 



107 S-ll Cs-Sb 



Lime or 
Borosil. 



108 S-13 Cs-Sb 



Fused 
Silica 



109 S-19 Cs-Sb 



Fused 
Silica 



110 S-20 



Na-K- 



Lime or 



Cs-Sb Borosil. 



Ill - 



Na-K- Lime or 
Cs-Sb Borosil. 



112 - 



Na-K- 
Cs-Sb 



UV Glass 



Range to Rants to Typical Range to Ranieto Apnroi 

Approx1% Appnn K% Maximum Approx10% Approx1% k S « QE 

Point Point Response Point Point | Im/W juA/lm mA/W % 






m 



-- j 



J»« •■!••• • • • 
■A*. I I Ti * * - 



HI! 



as 



EZ 






■ ■ ■ Tz-a 

• • • a '.v.-.vl 



F* « i • • » 
* • ■ ♦ • • ' 



u 



•TTinT 



n 



Point Point | 

^ to 1180 



rm rr»__ . 
» • • Li* • ►• 



1 



3 



m 



* thhti thpih ^ H ■*! nnnr! r Hr 



rrrr» 
l*JL*J 



113 - 



Na-K- 
Cs-Sb 



Pyrex 



114 
115 



Na-K- 
Cs-Sb 



Fused 
Silica 



"in nil 



Ei • • 
AM 



Err 
-JJ 



E 



7TT rrrr rrrr 



-* j 



Ti 



OZ 



Juj 



Itiu 



E 



► • • • 



E 



31 



iO 



E 






94 30 2.8 0.4 



1036 40 40 12.4 



800 60 48 13.2 



1253 40 50 18.2 



754 



3 2.3 O.i 



508 40 20 5.5 



803 70 56 15.7 



794 60 48 13.5 



1593 40 65 24.4 



429 150 64 18.8 



!iS!I 



in 

■dill 



295 300 89 20.8 



428 180 77 22.7 



385 200 77 23.9 



428 140 60 17.7 



- K-Cs-Sb 



Lime or 
Borosil. 



IE3 



D 



1190 67 79 24.5 



116 
117 
118 
119 
120 
121 



- K-Cs-Sb Pyrex 



m 



5D 



D 



1140 85 97 31.2 



- K-Cs-Sb UV Glass 






rrri 



1 



1190 67 79 24.4 



- K-Cs-Sb UV Glass 






rrrr 

lid 



ETTTTT 
Ml • »« < 



i 



to 930 
fV 

i nirt i 



1140 77 



27.2 



Na-K- 
Cs-Sb 



Pyrex 



•«t ••••••« 



^-Sb |L V p ge e 



• • ■ 
Lirl 



m 



— Cs-Te 



Fused 
Silica 



180 250 45 9.7 



1030 60 71 22 



11.6 7.2 



200 400 eoo no 

WAVELENGTH - NANOMETERS 



Technical Data 


















157 


SFEUTkAL 
RESPONSE 

CA EIA Cathode Window 
lode Desig- Material Material 
No. nation 


USEFUL SPECTRAL RANGE 


TYPICAL DATA 


Range to Range to Typical Range to Range to 
Approx1% Approx10% Maximum ApproxlO% ApproxlJS 
Point Point Response Point Point 


Approx. 
k S <r 0E 
Im/W ^A/lm mA/W % 


122 - K-Cs-Sb J 1 ™" 




















W 


• •4 


!• • • 


• • • 


M 


1040 83 87 26.9 














123 - Cs-Sb uv "g rade 
us - LSbD sapphire 




















f 7 










i 1 ^ 


800 60 48 13.2 




































124 - Cs-Sb UV Glass 


• • • 


• • • i 


1 !•'< 




Ji-vJ 


803 90 72 21.2 
















™ " Cs-Te lithium 




















IE 


• • • 


VI 


- - 11.5 6.1 










126 - K-Cs-Sb L ~ 




















rt 1- 


• • • • 


• ••• 


#j 


1140 72 82 27.4 










. 


127 - O 8 ^ 1 " UV Glass 




















• • • < 


• • • 1 


...I 


jv«J 


*" s X'i 


mi, 


o910 


410 60 25 6.2 


































128 .— • Ga-As "" UV Glass 


• • • 


i • • • 


• • • • 






• • • • 


• ••• 


133 250 33 7 




















129 - Ga-As-P UV Glass 


















280 200 56 17 


































1Qn Na-K- Lime or 
isa ~ Cs-Sb Borosil. 


|AV 


i • • • 




>••• 


• •• • 


* n*l-i= 


203 200 40 9 




































iai Na-K- Lime or 
iil ~ Cs-Sb Borosil. 


ti| 


i • •• 




r 


• • • •-•.•-•-• 


180 250 45 9.7 














,„ Na-K- Lime or 
" z ~~ Cs-Sb Borosil. 
















221 200 44 104 






L 








133 - K-Cs-Sb §g 


E 












1 

to 910 




• • • 


'•1 




>•• • 


N 


1190 60 71 22 














« " Ga-As >gg* 
















148 250 37 10.2 




















™ ~ GaAs-P «gjj£ 














n 




303 200 61 16 8 






































136 - K-Cs-Sb Lime 


i • • • 


•M 


!•'• 


» • • « 


-Jhi=Ml 


837 65 54 16.7 
















117 _ fia-As Lime or 
137 - ba-As Borosj| 




















• • • 


• • • 


■•*•".: 


•• • 


• • • 


105 330 35 7.9 














138 - lit UV6lass 














i 




• • • 


• • • 


• • • 






201 200 40 9 










r 







200 m too m 

WAVELENGTH - NANOMETERS 



: Conversion Factor 
: Luminous Sensitivity 



a = Radiant Sensitivity at Wavelength of Maximum 

Response 
QE = Quantum Efficiency at Wavelength of Maximum 
Response 



158 



RCA Photomultiplier Manual 



ELECTRON MULTIPLIER STRUCTURES 

Electron multiplier structures are I 
identical to those used in photo- 
multiplier tubes. Electron multipliers I 
are intended for use in vacuum sys- 
tems in the detection and measure- I 
ment of electrons, ions and other 
charged particles, as well as X-radia- I 
tion and vacuum ultraviolet radiation. 
The maximum average anode current I 
(30 second average) for all struc- 
tures listed in the chart below is 10 
microamperes. 



For Particle and Radiation Detec- 
tion 

For Use in a Vacuum System of 
10 -5 Torr, or Lower 

Broad Selection of Mechanical 
and Electrical Characteristics 

Various Types Feature an Integral 
Voltage Divider 

High Stability Copper-Beryllium 
Dynodes 



»ca 
PhotVaml- 

RCA tiplier Cage 
Oev. Tube Stricture 
ryue Witt (CuBa 
Ni. Similar Dyuoies) 

Djmede 
Structure 



Nnnatr 

of 
Dyaedes 



Outer 
Structure 





TYPICAL VALUES 




Voltage, 




Radiation 


Anode te 


Current 


Opeaiat 


Dynode 


Amplifica- 


laches 


Ne. 1, 


tion at 




Veils 


Typical 




(equal volts 


Cenaiiiip; 




per stage) 





C7875D 931A 



Circular 
Cage 



Flexible leads, 
10 sealed in 
bulb 



.31 x .94 



2000 

to 1 x 10" 
3000 



C71I7J 6810A In-Line 



Glass stem, 
,- (20) flexible 
14 leads (B20-102 

base supplied) 



.375 ± .010 X 
.375 ± .010 



3000 

to 

4000 



1 'to 10 X 10* 



C718.7K 6810A In-Line 



14 



Flange, glass 
stem, (20) 
flexible leads 
(B20-102 
base supplied) 



.375 ± .010 



3000 



^•°n!°n X T 1 to 10X10- 



4000 



C31017 8664 



Venetian 
Blind 



Stacked ceramic 
and Kovar 
construction, 
14 ring terminals, 
Flange mounting, 
shipped with 
sealed flange 



.900 x -730 



3000 

to 1 to 10 X 10" 

4000 



C31017A 8664 



Venetian 
Blind 



10 



Same as 
C31017 above 



.900 x .730 



2500 

to 1 to 10 x 10 5 

3500 



C31017B 8664 bim' 3 " 



10 



Stacked ceramic 
and non-magnetic 
material construc- 
tion, ring termi- 
nals, flange 
mounting 



.900 x -730 



2500 

to 1 to 10 x 10= 
3500 



Technical Data 



159 





RCA 
Paatenal- 










TYPICAL VALUES 












Valtaga, 




RCA 


tiplier 
fait 


Cue 


Nunbei 




Radiatiaa 


Aaede ta 


Carraat 


Dmr. 


Stractara 


of 


Oater 


Oaaaiai 


Dyaede 

Nl. 1, 


AMlifica- 
tfaaat 


Type 


Witt 


(CaBe 


Dyaodes 


Stractara 


lacaes 


Nl. 


Similar 
Dyaeae 

Stractara 


Byni(es) 








Valtt 
(eajal veRs 
par state) 


Typical 
Caaaitiaas 



Stacked ceramic 
u«««*;,n an d Kovar con ' 

C31017C 8664 !??„} 10 struction, ring .900 X -730 

mma terminals, flange 

mounting 



2500 

to 

3500 



Glass stem, 3grjg 

C31I1H4460 In-Line 14 {E^aled -250 ±.005 0. to 



leads, unsealed 
in bulb 



4500 



Support brackets, 

integral divider, 3900 

C31I1SB 4460 In-Line 14 (3) flexible .250 ±. 005 D. to 

leads, shipped 4500 

in plastic bag 



Integral divider, 3000 

C31I1SC 4460 In-Line 14 shipped in .250 ± .005 D. to 

plastic bag odou 



Kovar flange, 
glass stem, 
C31M1 4460 In-Line 12 (14) flexible .250 ±.005 0. 

leads, shipped 
in plastic bag 



C31I21A 4460 In-Line 



Glass stem, 
(14) flexible 
leads, shippe 
sealed in bulb 



C7.1I2F4460 In-Line 10 SJj^ -250 ±. 005 D. to. 1 X 10= 



3000 

to 

3500 



Kovar flange, 
glass stem, 



3000 



C7.1.2N4460 .,Line 10 JgJJj^ -250 ±. 005 0. ^ 

in plastic bag 



1 to 10 X 10 5 



Kovar flange, 

glass stem, 3900 

C31119 4460 In-Line 14 (16) flexible .250 d= .005 D. to 5x10" 

leads, shipped 4500 

in plastic bag 



5x10= 



5x^ 



1X10 3 



3400 

to 2.5 X 10 5 
4000 



Glass stem, (14) 3400 

12 flexible leads, .250 ±.005 0. to 2.5 X HF 

u shipped unsealed 4000 

in bulb 



1 X lO 1 



1 60 RCA Photomultiplier Manua 

ELECTRON MULTIPLIER STRUCTURES (cont'd) 



RCA 
Dev. 
Type 
No. 



RCA 
Photomul- 
tiplier 
Tube 
With 
Similar 
Dynode 
Structure 



Cage 
Structure 

(CuBe 
Dynodes) 



Number 

of Outer 

Dynodes Structure 



C70102K 4460 In-Line 



10 



Glass stem, 
(14) flexible 
leads, shipped 
unsealed in bulb 



Radiation 
Opening 
Inches 



.250 ± .005 D. 



TYPICAL VALUES 



Voltage, 

Anode to 

Dynode 

No. 1, 

Volts 

(equal volts 

per stage) 



Current 
Amplifica- 
tion at 
Typical 
Conditions 



3000 

to 1 to 5 X 10"' 

3500 



C70120E 8053 jjfj n n e d tian 



14 



(16) rod termi- 
nals, shipped in 
plastic bag 



.800 ± .010 D. 



3000 

to 1 to 10 x 10" 

4000 



C70129D 8571 $™ liT 



10 



Glass stem, 
(12) flexible 
leads, shipped 
unsealed in bulb 



.06 X .375 



2000 

to 1 X 10"' 

3000 



C70131 7850 In-Line 



12 



(13) tab termi- 
nals, shipped 
in plastic bag 



.375 ± .005 D. 



3000 

to 2.5 X 10' 1 

4000 



161 



Outlines 



THIS section shows the dimen- 
sional outlines for RCA photo- 
multipliers. See the Preliminary Se- 



lection Guides in the Technical Data 

Section for outlines and basing ref- 
erence numbers. 



1— i'.OZ— 1 
DIA. 



GLASS 
ENVELOPE -^ 

i_ 

.375 
MIN. 

T~ 

PHOTO-'' 
CATHODE 


-"1 




„ 


-.06 MlN 


"1 

1.24 

+.04 
-.05 

i 




^ 
^ 





1 
.43 
±03 

_L 

T4 

8ULB 


i 

I.3T 
MAX 


PROTECTIVE - 




i 


WAf-tK 1 V 






1 

1.5 
MIN. 

L 








II GOLD-PLATED 
-SEMIFLEXR3LE LEAD 
.0(6 t .004 DIA. 



■ 1 



INSULATING 

PROTECTIVE .,- 
WAFER f 




GLASS ENVELOPE 
II GOLD-PLATED 
SEMIFLEXieLE LEADS 
.016*. 004 MA. 



ULTRAVIOLET 
GRADE 
SAPPHIRE 
WINDOW 




U— It SEMIFUXWLE LEADS 
■ j020DIA. 1,5 LONG 



■3- 



162 



RCA Photomultiplier Manual 



PHOTOCATHODE- 



PERMANENT 8ASE 

JEDEC NO.E12-72 

AND PROTECTIVE 

SHELL 



rMIN. 
DIA. 



n 



JEI 



12SEM1FLEXIBLE LEADS 



PHOTOCATHODE - 



PERMANENT BASE 
JEDEC No. E12-72 
AND PROTECTIVE 

SHELL 
(ON C3I005B ONLYJ v 



"~MIN. | 

... ] 



jprtn 



h 



4460 

C70102B 

C70102E 

C70102M 

C70102N 


4516 
7767 
C70042J 


8644 
C70042D 
C70O42K 
C70042R 


A-3.38 Max. 

B-2.94 tjl 

C-0.30 Max. 
D-0.38 Max. 


194 Max. 

3.50 +■« 

0.30 Max. 
0.38 Max. 


3.8 Max. 

3.26 ± 0.15 

0.28 Max. 
0.30 Max. 



■4- 



C31005 


C31005B 


A-3.7 Max. 

B-3.26 1'^ 

C-0.87 Max. Dia. 
D-0.5 Max. 
E- 


3.94 Max. 
3.50 +;J| 
0.755 Max. Dia. 

0.38 Max. 



-6- 



.735' 

-H ±.020" f*- 

i °" r 



PHOTOCATHODE - 



SMALL-BUTTON 
NINAR 9-PIN BASE 
JEDEC N* E9-37 — 



«-.78"V 




■7- 



Outlines 



163 



^_oa MIN. . 



— PHOTOCATHODE 




24 MIN. -05- 



PHOTCCATHODE- 




14 HATED 

SEMIFLEXIBLE LEADS 
.028 £004 DIA. 
MIN. LENGTH* 1.5 



—VOLTAGE 
DIVIDER 
NETWORK 



-10- 




PHOTOCATHOOC 



INSULATIW 
PROTECTIVE 
WAFER 




FOCUSING 

ELECTRODE 

LEAO 




1.31 MAX. «.„. 



z^sr 



13 GOLD-PLATED 
SEMIPLEXIBLE LEADS 
~ £16 ±.004 DIA. 
LEAD LENGTH 1.5 MIN. 





1P21 






1P22 






1P21 






931 » 






4471 






4472 


1P2S/V1 




4473 


1P2S/V1 




C31004 


C7075J 




1P2IA 




A- 


1.94 ± 0.09 


1.99 ± 0.09 



-11- 



164 



RCA Photomultiplier Manual 







»___ i.ie . 

MAX. DIA. 










T» BULB ^ 


£ 






.31 MIN. 




T 
i 


—1 1— 


—.22 

~r 








\ 




> 










METAL 
ENVELOPE — 

BASE _ 
JEDECNo.BlhBB 


^u_ 


3.4 


t 




M 


IX 




I 






3.125 
MAX. 




t 


1.56 
+.09 
















SMALL- SHELL 
NEOSUBMAGNAL 
II-P1N BASE * 




M* 


X. 








EOEC N*BII-I04 


uipii 








DI 

.31 


jyijuu 




. 1.31 






-^ 


MAX 


DIA. 


„ . 











-12- 



■15- 




■-.200 

1] FLEXIBLE LEADS 



-13- 



FHOTOCATHOOt 




BASE 
JEDE0WBI1-M 




BASE « 

JEOEC No. BII-B8 



C31025B 
C3102SG 


C31025C 


A-0.31 Min. 
B-0.94 Min 


0.2 Min. 
0.6 Min 



-16- 







Urn — 
MA 


X. DIA. 




1. 

2 

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3.10 
MAX. 






1 


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+.07 
-.12 

1 




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i .015 



12-PIN 

DUODECAR- 

BASE 



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165 




12 FLEXIBLE 

LEADS 1.3 

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a. si 

±.06 



12 SEMI FLEXIBLE 

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12 SEMIFLEXI9LE 

LEAOS 
.020 ±. 003 DIA 
MIN. LENGTH 



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PHOTOCATHODE- 



7" 



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El2 



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1— '-2« — < 
^ MIN. DIA. ^ 



PWTOCATHOOE- 
TI2 BULB 



■19- 



12 GOLD-PLATED 

SEMIFLEXIBLE LEADS 

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LEAD LENGTH 2 MW. 






-22- 



166 



RCA Photomultiplier Manual 



PHOTOCATHOOE 
.75 MIN. R. 




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BASE 
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No.BW-38 
2.31 MAX. DIA. 



■27- 



-29- 



168 



RCA Photomultiplier Manual 



PHOTOCATHCOE 



BASE 

JEDEC GROUPS 
NO.BM-3B 





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B342A, E655A, 




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$903 


A-5.2 Max. 


5.81 Max. 


6-9/16 Max. 


B-4.25 ± 0.19 


4.87 ■+• 0.19 


5-5/8 ■+■ 3/16 


C-2.00 ± 0.06 


2.00 ■+• 0.06 


2 -+- 5/32 Dia. 


Dia. 


Dia. 




D-1.68 Min. 


1.68 Min. 


1-5/8 Min. Dia. 


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Flat 



FACEPLATE — 



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DIHEPTAL 

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No. BI4-38 



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9 STRANDS No. 36 
-ENGTH 29 



35- 



170 



RCA Photomultiplier Manua 



FACEPLATE— - 



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3.00 ± .06 . 
~ DIA, 

2.99 MM. 
— DIA. " 



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-MATES WITH BNC U9-86C/U 
OR EQUIVALENT. 



-39- 



Outlines 



171 



FACEPLATE' 



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.0301.005 DIA. 
LEAOLCNOTHUMN. 



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-41- 



172 



RCA Photomultiplier Manual 




- 5.30±.O6DIA. - 
-5.00±.06D1A.- 




2 EXHAUST 

TUBULATIONS 
(I20"APART) 



-43- 



Outlines 



173 




-44- 



174 



Basing Diagrams 



This section shows the basic dia- 
grams for RCA photomultipliers. See 
the Preliminary Selection Guides in 
the Technical Data Section for out- 
line and basing reference numbers. 
Temporary bases are given at end 
of this section. 



FLEXIBLE ENVELOPE 
TERMINAL 






RIGID ENVELOPE TERMINAL 



ORIENTATION SYMBOL 
OTHER THAN KEY 



Key to Terminal Connections on Basing Diagrams. 



8571 C70129G* C70129H* 



s*\ ±20* tWl*--. 

orA If 




4460 4516 7767 C70042J" 
C70102E" C70102M° C70102N" 



-1. -2- -3- 




8844° 


C7N42D° 
C7U42R° 


C70042K 




P "So w 9 




w, 6> 




~© m * 


W 7© 

■"sd 




~®f. 


DY 3 







C70112B* 




-4- 



' Supplied with temporary base A. 



' Supplied with temporary base B. 



Basing Diagrams 



175 





C31016B* 




C31005+ C70128+ 





■6- -7- 



■ 10- 





■Supplied with temporary base C. 



°YI t W K 

DIRECTION OF RADIATION 

I 

-11- -12- 

• Supplied with temporary base D. 



176 



RCA Photomultipiier Manual 




-13- 




-14- -15- -16- 




17 




•18- 



4441 4441 A 4461 A 




-19- 
C70114C C70114E" 




-19- 



4517 6199 7192 C71S1W 




-18- 

' Supplied with temporary base B. 



C70114D" C79114F" 
C79132B° 


C701HJ 


m 9 A o»ia 




(t) V © 


D Y 8 
Q) 


" 5 ®-/-^ £— V 


J® DY 6 




DY 2 



-19- -20- 



Basing Diagrams 



177 



2067° C70132A- 



C71510." 





-20- 



■22 




4526° 



-21- 




-23- 



4440 





■24- 




-22- 

' Supplied with temporary base B. 




• 25- 



178 



RCA Photomultiplier Manual 





-27- 



4463 4523 8053 
C70109E 




-29- 




-30- 





-28- 



■31- 



681 0A 7264 



20634: 

or, AS ¥ 4" 




-29- 

t Supplied with temporary base E. 




-32- 



Basing Diagrams 



179 



7265 C7268 



or,® 





NC fir N" 



-36- 



-32- 



0, s 


OY 7 


OYfl 


D w 




**Viq)Dy| 


o^gKi 


^f^P 


DY 3 (DC 




/\y^t2)IC(DO NOT USE) 


DY,^ 


pST^- n- metal 
vz> ** collar 

K IC 






(DO NOT USE) 



-33- 




-37- 




"O Ic SHKLO 



4464 4524 8054 




-34- 



•38- 



2064B* 



Finns provide 
electrode cemectiMs 




-35- 



-38- 



t Supplied with temporary base E. 



180 



RCA Photomultiplier Manual 



Request technical data 

sheet for electrode 

connection information 



-39- 




-41- 



45214: 



DV 6 0Y 7 




-40- 



4525 8055 




Flanges provide 
electrode connections 



-42- -43- 



METAL PORTION OF 
TUBE ENVELOPE 




-41- 



-44- 



4465 




IC (DO NOT USE) 



-41- 

$ Supplied with temporary base E. 



DY — Dynode 

G —Grid 

IC — Internal connection 

(do not use) 
NC — No connection 
P — Anode 
K — Photocathode 



Basing Diagrams 



181 



TEMPORARY BASES 





-A- 




DY7 DV 8 
DY 6 _ ffl CS) J V > 




f> K °w 



-E- 




DY — Dynode 

G — Grid 

IC — Internal connection 

(do not use) 
NC — No connection 
P — Anode 
K — Photocathode 



-C- 



182 RCA Photomultiplier Manual 



183 



Index 



Absorption spectroscopy 100 

Acceptor level 8 

Afterpulsing 47 

Anodes 24, 25 

Atmosphere ••••• 36 

Band gap 7 

Bandwidth, noise 68 

Basing 27 

Bialkali photocathode 10, 11 

Black-body radiation 119 

Box-and-grid multiplier structure 23 

Cathodes 

bialkali 10, 11 

ERMA 10, 11 

multialkali 11 

opaque 10, 19 

semitransparent 10, 19 

trialkali 11 

Cerenkov radiation 77, 86 

Charge integration 93 

Collection efficiency 20 

Compression, signal 110 

Conduction band 7 

Conversion 

of units 112 

Counting 46, 51 

interpretation 89 

photon 95 

Current control 55 

Current protection 110 

Current rating 

anode current 32 

cathode current 31 

Dark current 19, 40 

equivalent anode dark current input 

(EADCI) 42 

reduction 43 

Dark noise 44, 45, 66 



Data, classes of 30 

Densiometry 99 

Detection 

digital method 94 

low-light level 93 

photon counting 95 

Doping 9 

Dynamic compression 110 

Dynode 5, 6 

box-and-grid 21, 23 

EADCI 43 

Electric field, exterior 34 

Electron affinity 7 

Electron multipliers 21, 26 

time response 26 

Energy band 7, 8 

Energy sources 128 

ENI 43 

Environment 35 

atmospheric 36 

shock : 35 

temperature 32 

vibration 35 

Equivalent Noise Input (ENI) 43 

Extended Red Multi-Alkali 

(ERMA) photocathode 11, 12 

Eye characteristics 112 

Fatigue 49, 50 

Fermi level 7 

Fluctuations 56, 64 

Flying leads 27 

Flying-spot scanning 90 

CRO 91 

objective lens 92 

photomultiplier 92 

Forbidden band 7 

Gas ionization 66 

Hysteresis 51 



184 



RCA Photomultiplier Manual 



Integration, charge 93 

Lamps 120 

Laplace equation 19, 24 

Lasers 121 

-range finder 89 

Life expectancy 52 

Light detection 93 

photon counting 95 

Light feedback 67 

Light sources 121 

Linearity _ _ 53 

Logarithmic photometry 99 

Magnetic field 34 36 

Matching 

calculations 130 

-factor 132 

source-detector 127 

Multiplier analysis 24 

Multipliers 

contribution to noise 61 

for scintillators 82 

structures 22 23 

Negative electron affinity 8 

Noise 

-and bandwidth 68 

dark 66 

gas ionization 66 

group I and II sources 66 

multiplier contributions 60 

photocathode contributions 57 

photon 56 

statistics for dynodes 61 

Output-current control 55 

Parameters, design 30 

Peak-to-valley ratio 46 

Photocathode 6 19 

contribution to noise 57 

opaque 10 

reflection at- 54 

ruggedized 26 

semitransparent 10 

sensitivity 9 38 

spectral response 39 

transit-time difference 25 

Photoemission 5 

time lag 16 

Photometry 97 

color-balancing 98 

densiometry 99 

logarithmic 99 

units 113 

Photomultiplier 5 19 37 



applications 77 

characteristics 82 

current protection no 

data 37 

design 19 

-for photometry 97 

-for photon counting 96 

-for Raman spectroscopy 102 

-for scanner 92 

sensitivity 37 

testing 126 

Photon noise 56 

Power supply no 

high-voltage 1 10 

stability m 

Pulse counting 46, 106 

Pulse height 

distribution 63 

resolution 46 

Quantum efficiency 40 

Radiant energy n9 

sources 120 

Radiation 

black-body 1 19 

sources 120 

Raman spectroscopy 101 

Ratings 30 

anode current 32 

cathode current 31 

temperature 32 

voltage 30 

Reflected light 54 

Regenerative effects !....". 41 

Ruggedization ™ 26 

Scintillation 

counter 77 80 

materials 8 o[ 83 

mechanism 79 

processes -jg 

spectrometer 84 

Secondary emission 5, 13 61 

statistics, effect of 16 

time lag i 6 

Sensitivity 9 37 

blue 38 

integral radiant 38 

lo ss "".'.' 50 

luminous 38 

radiant 38 

Shock 35 

Signal compression no 

Source-detector matching 127 129 

Spacial uniformity 55 

Spectral response 10, 39 127 

Spectrometry iqq 

absorption 100 

Raman iqi 



Index 



185 



Spectroscopy 

energy 84 

time 87 

itability 49, 51 

Statistical fluctuations 56 

Temperature rating 32 

Time-domain reflectometry (TDR) 25 

Time lag 16 

in photoemission 16 

in secondary emission 16 

Time response 19, 26, 48 

electron-multiplier 26 

Time spectroscopy 87 

Time stability test 52 

Transit time 25 

-spread 49 



Uniformity, spacial 55 

Valence band 7 

Venetian-blind multiplier structure 22 

Vibration 35 

Video amplifier 93 

Voltage divider 103 

tapered 109 

-for pulsed operation 106 

-resistors 104 

voltage ratio 104 

wiring 107 

Voltage rating 30 

Wiring techniques 107 



186 RCA Photomultiplier Manual 



Contributors 187 



LIST OF MAJOR CONTRIBUTORS TO THE 
RCA PHOTOMULTIPLIER MANUAL 

Ralph W. Engstrom 
Fred A. Helvy 
Harold R. Krall 
Thomas T. Lewis 
W. Dean Lindley 
Ramon U. Martinelli 
Robert M. Matheson 
Anthony G. Nekut 
Dennis E. Persyk 
Ronald M. Shaffer 
Albert H. Sommer 



188 RCA Photomultiplier Manual 



189 



Other RCA Technical Manuals 



• RCA POWER CIRCUITS— SP-51— 

(8" x 5V4") — 448 pages. Contains design in- 
formation for a broad range of power 
circuits using RCA silicon transistors, rec- 
tifiers, and thyristors (SCR's, triacs, and 
diacs). Gives design criteria and procedures 
for applications involving rectification, sup- 
ply filtering, power conversion and regula- 
tion, ac line-voltage controls, rf power 
amplifiers, and control and low frequency 
amplifiers. Shows design examples and prac- 
tical circuits. Price $2.00.*t 

• RCA TRANSISTOR, THYSISTOR & 
DIODE MANUAL— SC-14 (8" x 5Va")— 

656 pages. Contains up-to-date definitive 
data on over 700 semiconductor devices 
including tunnel diodes, silicon controlled 
rectifiers, varactor diodes, conventional rec- 
tifiers, and many classes of transistors. 
Features easy-to-understand text chapters, 
as well as tabular data on RCA discon- 
tinued transistors. Contains 38 practical 
circuits, complete with parts lists, high- 
lighting semiconductor- device applica- 
tions. Price $2.50.*t 

• RCA HIGH-SPEED, HIGH-VOLT- 
AGE, HIGH-CURRENT POWER TRAN- 
SISTORS— PM-80 (8" x 5W')— 96 pages. 
Provides a basic understanding of the 
theory and application of the RCA line of 
medium-frequency power transistors. Cov- 
ers physical theory, structures, geometries, 
packaging, critical application-limiting fac- 
tors, and the operation and requirements of 
power transistors in amplifier, switching, 
and control applications. Typical circuits 
illustrate the use of transistors in series 
voltage regulators, linear amplifiers, switch- 
ing regulators, and inverters and con- 
verters, and the application of complemen- 
tary transistor pairs. Selection charts are 
included to facilitate choice of the optimum 
type of power transistor for a variety of 
military, industrial, and commercial appli- 
cations. Price $2.00.*t 



• RCA SOLID-STATE HOBBY CIR- 
CUITS MANUAL— HM-91 (8%" x 5%") 
— 368 pages. Contains complete construc- 
tion information on 62 circuits of general 
interest to all experimenters. Circuits use 
diodes, transistors, SCR's, triacs, MOS 
transistors, integrated circuits, and light 
and heat detectors. Circuit operation is 
described in detail; construction layouts, 
photographs, schematic diagrams, and parts 
lists are given; and full-size drilling or 
printed-circuit templates are included for 
most circuits to simplify construction. 
Price $1.95.*t 

e RCA SILICON CONTROLLED REC- 
TIFIER EXPERIMENTER'S MANUAL 

— KM-71 (iW x 5W'h- 136 pages. Con- 
tains 24 practical and interesting control 
circuits that can be built with a comple- 
ment of active devices available in kit 
form. Includes photographs, schematic dia- 
grams, and descriptive writeups. Also in- 
cludes brief descriptions of solid-state 
components used (rectifiers, transistors, 
SCR's) and short section on trouble- 
shooting. Price 95 cents.*t 

• RCA RECEIVING TUBE MANUAL 

— RC-27 (8" x 5J4") — 672 pages. Contains 
technical data on more than 1400 receiv- 
ing tubes for home-entertainment use. In- 
cludes six easy-to-read text chapters that 
provide basic information on electron-tube 
operation, ratings and characteristics, and 
applications. Also features a detailed ap- 
plication guide for receiving tubes; quick- 
reference charts for replacement and 
discontinued RCA receiving tubes, black- 
and-white and color picture tubes, and 
voltage-reference tubes; and a Circuits sec- 
tion that includes schematic diagrams, de- 
scriptive writeups, and complete parts lists 
for 36 practical electron-tube circuits for 
a wide variety of applications. Price 
$2.00.*t 



190 



RCA Photomultiplier Manual 



• RCA TRANSMITTING TUBES— TT-5 

(8%" x 5%")— 320 pages. Gives data on 
over 180 power tubes having plate-input 
ratings up to 4 kW and on associated rec- 
tifier tubes. Provides basic information on 
generic types, parts and materials, installa- 
tion and application, and interpretation of 
data. Contains circuit diagrams for trans- 
mitting and industrial applications. Fea- 
tures lie-flat binding. Price $1.00*f 

• RADIOTRON" DESIGNER'S HAND- 
BOOK— 4th Edition (fi 3 A" x 5Vi")— 1500 
pages. Comprehensive reference covering 
the design of radio and audio circuits and 
equipment. Written for the design engineer, 
student, and experimenters. Contains 1000 
illustrations, 2500 references, and cross- 
referenced index of 7000 entries. Edited 
by F. Langford-Smith. Price $7.00.*t 

• RCA PHOTOMULTIPLIER MANUAL 

— PT-61 (8'/4" x 5'/4")— 192 pages. The 
construction, operation, and application of 
photomultipliers are detailed in this well- 
illustrated manual. Discussions of photo- 
emission, secondary emission, and the 
design of multiplier structures are followed 
by a description of photomultiplier per- 
formance characteristics. An analysis of 
statistical fluctuation and noise effects then 
leads into a section on applications of 
photomultipliers. Photometric and radiant 
units, radiant energy sources, spectral re- 
sponse, and source-detector matching are 
set forth. An extensive tabulation of tech- 
nical data includes a selection guide for 
over one hundred RCA photomultiplier 
tubes, with a special section on ruggedized 
types. Outlines, basing diagrams, and cir- 
cuit information are furnished. A complete 



index adds to the convenience of this man- 
ual. Price $2.50.*t 

• RCA ELECTRO-OPTICS HAND- 
BOOK— EOH-10 (8'/ 2 " x 5W)— 154 
pages. Data from the many technical areas 
involved in electro-optics are compiled and 
unified in this manual. The presentation is 
convenient, featuring more than one hun- 
dred tables, charts, and graphs. A coherent 
system of symbols and definitions is listed, 
and data on radiometry, blackbody radia- 
tion, and photometry are presented. Infor- 
mation on radiant sources, lasers, at- 
mospheric transmittance, and detection, 
resolution, and recognition are included. 
Treatments of detector characteristics, 
image and camera tubes, and optics are 
followed by a section on electro-optics 
systems analysis. References and an ex- 
tensive index add to the usefulness of this 
unique handbook. Price $2.50.*t 

• RCA LINEAR INTEGRATED CIR- 
CUITS— IC-42 (8'4" x 5%")— 416 pages. 
Explains the basic principles involved in 
the fabrication, design, and application of 
linear integrated circuits. Includes a dis- 
cussion of the basic silicon monolithic fab- 
rication process, analyses of the building- 
block elements used in linear integrated 
circuits, explanations of the operation and 
applications of RCA linear integrated cir- 
cuits, and detailed ratings and characteris- 
tics data and package information for 
RCA linear integrated circuits. Price 
$2.50.*t 

"Trade Mark Reg. U.S. Pat. Off. 
•Prices shown apply in U.S.A. and are 
subject to change without notice. 
T Optional price. 



pectral Response Characteristics 



191 



RCA TYPICAL PHOTOCATHODE SPECTRAL 
CHARACTERISTICS (cont'd) 


RESPONSE 




Spectral 
Response 


Cathode 
Material 


Window 
Material 


Trans- 
mission 
Tj?e 


Opaque 
Type 


Sensi- 
tivity 
fiA/lm 


Sensi- 
tivity 
mA/W 
@ \ max. 


Uuantum 
Efficiency 

% 
@ \ max. 


Dark 
Emission 
at 22° C 
fA/cm** 
(1fA = 
1 x 10-« A) 


117 


K-Cs-Sb 


uv 

Glass 


X 


— 


67 


79 


24.4 


0.02 


118 


K-Cs-Sb 


UV 
Glass 


X 


— 


77 


88 


27.2 


0.02 


119 


Na-K-Cs-Sb 


Pyrex 


X 


— 


250 


45 


9.7 





120 


K-Cs-Sb 


UV-grade 
Sapphire 


X 


- 


60 


71 


22 


- 


121 


Cs-Te 


Fused 
Silica 


X 


— 


— 


11.6 


7.2 




122 


K-Cs-Sb 


Aluminum 
Oxide 


X 


— 


83 


87 


26.9 




123 


Cs-Sb 


UV-grade 
Sapphire 


— 


X 


60 


48 


13.2 




124 


Cs-Sb 


UV 
Glass 


X 


— 


90 


72 


21.2 




125 


Cs-Te 


Lithium 
Fluoride 


X 


— 


— 


11.5 


6.1 




126 


K-Cs-Sb 


Lime or 
Borosil. 


X 


— 


72 


82 


27.4 


" 


127 


Ag-Bi-0-Cs 


UV 
Glass 


X 


— 


50 


34 


8.4 




128 


Ga-As 


UV 
Glass 


— 


X 


200 


35 


7.9 


0.1 


129 


Ga-As-P 


UV 
Glass 


— 


X 


200 


61 


16.8 


0.01 


130 


Na-K-Cs-Sb 


Lime or 
Borosil. 


X 


— 


200 


40 


9 




131 


Na-K-Cs-Sb 


Lime or 
Borosil. 


X 


— 


250 


45 


9.7 




132 


Na-K-Cs-Sb 


Lime or 
Borosil. 


X 


— 


200 


44 


10.4 




133 


K-Cs-Sb 


Fused 
Silica 


X 


— 


60 


71 


22 


" 


134 


Ga-As 


UV-grade 
Sapphire 


— 


X 


200 


35 


7.9 


" 


135 


Ga-As-P 


UV-grade 
Sapphire 


— 


X 


200 


61 


16.8 


0.01 



192 



RCA Photomultipiier Manu; 



RCA TYPICAL PHOTOCATHODE SPECTRAL RESPONSE 
CHARACTERISTICS 



The Chart of RCA typical photo- 
cathode spectral response character- 
istics given on the inside front and 
back covers of this Manual contains 
curves representing many of the 
photomultiplier-tube spectral response 
characteristics currently available 
from RCA. These curves do not show 



RCA's complete capability and, f< 
clarity reasons, only the most con 
mon are illustrated. 

The table below summarizes tl 
typical characteristics of the phot< 
cathodes indicated on the spectn 
response chart. 



Spectral 
Response 


Cathode 
Material 


Window 
Material 


Trans- 
mission 
Type 


Opaque 
Type 


Sensi- 
tivity 

jiA/lm 


Sensi- 
tivity 

IRA/W 

@ X max. 


Quantum 
Efficiency 

% 
@ X max. 


Dark 
Emission 
at 22°C 
fA/cmi* 
(1fA = 
1 x 1«- M A) 


101 
(S-1) 


Ag-0-Cs 


Lime or 
Borosil. 


X 


X 


30 


2.8 


0.43 


900 


102 

(S-4) 


Cs-Sb 


Lime or 
Borosil. 


— 


X 


40 


40 


12.4 


0.2 


103 


Cs-Sb 


UV 
Glass 


— 


X 


60 


48 


13.2 


0.3 


104 

(S-5) 


Cs-Sb 


UV 
Glass 


— 


X 


40 


50 


18.2 


0.3 


105 
(S-8) 


Cs-Bi 


Lime or 
Borosil. 


— 


X 


3 


2.3 


0.78 


0.13 


106 
(S-10) 


Ag-Bi-0-Cs 


Lime or 
Borosil. 


X 


— 


40 


20 


5.5 


70 


107 
(S-11) 


Cs-Sb 


Lime or 
Borosil. 


X 


— 


70 


56 


15.7 


3 


108 
(S-13) 


Cs-Sb 


Fused 
Silica 


X 


— 


60 


48 


13.5 


4 


109 
(S-19) 


Cs-Sb 


Fused 
Silica 


— 


X 


40 


65 


24.4 


0.3 


110 
(S-20) 


Na-K-Cs-Sb 


Lime or 
Borosil. 


X 


— 


150 


64 


18.8 


0.4 


111 


Na-K-Cs-Sb 


Lime or 
Borosil. 


— 


Notel 


300 


89 


20.8 


0.4 


112 


Na-K-Cs-Sb 


UV 
Glass 


X 


— 


180 


77 


22.7 


0.4 


113 


Na-K-Cs-Sb 


Pyrex 


X 


- 


200 


77 


23.9 


0.4 


114 


Na-K-Cs-Sb 


Fused 
Silica 


X 


- 


140 


60 


17.7 


0.4 


115 


K-Cs-Sb 


Lime or 
Borosil. 


X 


— 


67 


79 


24.5 


0.02 


116 


K-Cs-Sb 


Pyrex 


X 


- 


85 


97 


31.2 


0.02 


Chart Continued an Preceding Page. 















* Based on Dark Current Measured in Typical PMT's. 
Note 1: Reflective Substrate